BMX Race Gearing and Roll-Out: The Complete Guide

By Bill Ryan — Founder of Supercross BMX · Former Technical Editor, BMX Plus Magazine · Workshop columnist, GO Magazine · Designer of 8× Bike of the Year race frames · 37+ years in BMX

Every week somebody emails me a chainring and cog number and asks if it's right. Forty-four sixteen. Forty-five seventeen. Thirty-nine fourteen. And every week the honest answer is that those two numbers by themselves don't tell either of us anything useful.

What matters is roll-out: how far your bike travels down the track for one full turn of the pedals. That's the number your legs actually feel on the gate, and it's the only one that lets you compare a gear against another gear, a bike against another bike, or your setup against a teammate's. Two gears that look nothing alike on paper can ride almost identically. Two that look close can be miles apart.

There are eight full gear charts further down this page, covering every wheel and tire size we sell parts for, and every one of them is centered on the same baseline number. Before you scroll to them, spend two minutes on the formula. Once you can run it yourself you don't need anybody's chart, including mine.

Start with the formula

The chainring turns the chain, the chain turns the cog, the cog turns the rear wheel. How far you travel for one pedal stroke comes down to three things and nothing else: chainring size, cog size, wheel size.

Roll-out (inches) = (chainring ÷ cog) × wheel diameter × π

π is 3.1416. All it does is turn a wheel's diameter into how far that wheel rolls in one complete turn.

Work a 44/16 on a 20-inch wheel. Forty-four divided by sixteen is 2.75, so the wheel turns 2.75 times for every turn of the cranks. Multiply 2.75 by 20 and then by 3.1416 and you get 172.8 inches, a little over fourteen feet per pedal stroke. That's the whole thing. A parent standing in the pits can run it on a phone in twenty seconds.

Do it once with your own numbers before you read another word. Everything else on this page is that one line of arithmetic applied to different wheels, different cranks and different tires.

Two numbers, both called roll-out

Here is the thing that trips people up, and nobody explains it, so I will.

Walk the staging lanes at a USA BMX national and ask what people are running. You'll hear "fifty-five." Look at a British or Australian club page and roll-out is a number like 173. Both are right, and they are the same gear. The 55 is the ratio times the wheel diameter and stops there, which is the old bicycle measure called gear inches, and American BMX has called it roll-out for forty years. The 173 keeps going and multiplies by π to get the real distance rolled. One is the other times 3.1416. The only mistake is comparing a 55 to a 173 and thinking you're looking at two different gears.

Every chart on this page gives you both numbers, so you never have to convert anything. The big number in each cell is gear inches — ratio times wheel diameter, what an American racer means when he says "I run 55." The small grey number underneath is roll-out — that same gear times π, the true distance the bike travels for one turn of the cranks, which is what UK and Australian charts quote and what you get when you take a tape measure out to the driveway. Same gear, two languages, side by side in every cell. Our earlier piece, The Great Debate: BMX Gearing, works entirely in gear inches if that's the language you grew up with.

The 55-inch club

There is one number running underneath this entire sport and hardly anybody says it out loud. It is 55.

A 44/16 on a 20-inch wheel is 2.75 to 1. Times 20 inches of wheel, that is 55.00 gear inches, dead on. That is the BMX race standard. It is the gear that comes on most complete bikes, and it is what the staging lane means when somebody says he runs fifty-five.

Now look at what freestyle does. Park riders run micro-drive, a small ring and a tiny cog, so the sprocket sits up out of harm's way and survives grinds and hard landings. The park standard is 25/9. That is 2.7778 to 1. On the same 20-inch wheel, 55.56 gear inches.

And my own gear, the 47/17 nobody else ran, is 2.7647 to 1. Fifty-five point two nine.

44/16 and 25/9 are 0.56 gear inches apart. That is 1.01%.

Sit with that a second. The race side of BMX arrived at 44/16 over forty years of gate starts. The freestyle side worked the problem completely separately, and was not even chasing the same thing — they wanted a sprocket small enough to survive a grind, not a faster gate. They landed on 25/9. Put those two gears on paper and they look like they have nothing to do with each other. Twenty-five and nine against forty-four and sixteen. They are the same gear, inside one percent. My 47/17 sits right between them.

Three generations of riders, three answers that look nothing alike, all circling the same number. Nobody coordinated it. It happened because 55 gear inches is about what a human being can actually turn over out of a gate on a 20-inch wheel. That number was never a convention somebody agreed to. It is the answer, and everybody found it from a different direction.

That is why 55 is the center of every chart on this page. Each chart is built around the chainring that lands closest to 55 gear inches on that wheel size, and the yellow cells mark the 55-inch band. Once you know where 55 sits for your wheel, every gear on your bike has a reference point. You are either geared up from it or geared down from it, and the chart tells you by exactly how much.

Here is the practical version. If you are coming off a park bike onto a race bike, or moving from a 20 to a cruiser, do not copy somebody else's tooth count. Find your 55 on the chart for your wheel size and adjust from there. On a 24-inch cruiser that is a 37 on a 16. On a 26 it is a 34. On a 29 it is a 30. Same gear every time, different teeth, because the only thing that changed was the wheel.

The 47/17 and the 36/16

Now the part I actually wanted to write.

"I personally love a 47/17 on a 20" and a 36/16 on a 24" race bike, which I know seems odd, but they always felt the closest to me when jumping from bike to bike. This was based upon my original Supercross BMX Frame, the last bike I actually raced, and my Bracken's Prototype Cruiser which I designed with Tommy Brackens and when Kevin Hull and I wanted to race Cruisers."

Those are my gears and they have been my gears since the late 1980s. The 47/17 came off my original Supercross BMX frame, the last bike I raced myself. The 36/16 came off my Brackens prototype cruiser, which I designed with Tommy Brackens back when Kevin Hull and I decided we wanted to race cruisers.

Here is the full setup, because the gear numbers on their own are only two thirds of the story. The 20 ran 47/17 on an Odyssey Renegade 20 x 1.75 rear tire with 182mm cranks. The Renegade is a tire I drew up for Odyssey in the mid 1980s, so I knew exactly what that casing did under load and how tall it sat. The cruiser ran 36/16 on an IRC X-1 24 x 1.75 with 180mm cranks.

Nobody ran 47/17. A 47-tooth ring wasn't a common part, and a 17 cog was for riders trying to make a gear lighter, not heavier. Put 47/17 and 36/16 side by side on paper and they look unrelated. For years I couldn't explain why those two bikes felt like the same bike underneath me. I just knew they did, and I quit arguing about it. Here's what was actually going on.

Note where the 20 lands on the gear-inch row: 55.3. That is the 55-inch club again, sitting between the 44/16 at 55.00 and the freestyle 25/9 at 55.56. I did not know that when I picked it. I picked it because it felt right.

Read that bottom row twice. It's the reason this whole article exists.

Roll-out lands within 2.34%. On its own that would be a nice coincidence. But the leverage at the pedal, which is a completely separate calculation involving crank length and wheel radius, lands within 1.26%. Both measures agree. The two bikes take the same distance per stroke and the same effort to turn that stroke over.

I wasn't matching gear numbers. I was matching roll-out and lever feel, by seat of the pants, decades before anybody in BMX was doing this arithmetic. My body found the answer and the calculator confirmed it thirty-some years later. That's not me being clever. That's a rider spending enough hours on both bikes that the difference between "close" and "right" stopped being a guess.

If you've ever hopped off your 20 onto your cruiser and felt like you were suddenly pedaling through wet sand, that's the whole reason, and it's ten minutes with a cog wrench to fix.

The gear chart on the Spin Drive box

Long before this page existed, we were already printing gear charts.

In our original Spin Drive hubs, we put a BMX gear chart on the inside flaps of each side of the box. It was a great reason to save the box. You got a 20 x 1.75 chart, a 24 x 1.75 chart, a 20 x 1 1/8" chart and a 20 x 1 3/8" chart, sitting in the garage where you actually needed them, next to the bike with the wrench in your hand. I still run into people who kept those boxes for twenty years for exactly that reason. The hub was the product. The box was the reference.

Those four charts covered everything that mattered in the early 1990s. They don't cover what people ride now. Big BMX is real, 29-inch bikes are real, oversize 20s are real, and freestyle runs gearing that has nothing to do with race gearing.

So consider this page the re-issue. The original four are here, plus 26 x 2.125 for Big BMX and retro cruisers, 29 x 2.3 for the big bikes, OS20 x 1.65 for oversize 20s, and 20 x 2.3 for freestyle and park. Eight charts, one page, free, on a phone in the pits instead of a cardboard flap in the garage.

Credit where it's owed: Shane Jenkins and bmxultra.com

Before you scroll into my charts, go look at his.

Shane Jenkins has built and maintained the most complete BMX gear chart in the world over at bmxultra.com. Not the most complete one from an Australian site. The most complete one, period. He has a gear calculator, a reverse lookup so you can start from the roll-out you want and work back to the teeth, a decimal chart, a version for big gears, and phone apps. He's kept it current and kept it free for the better part of two decades.

That's a lot of unpaid nights for a sport that mostly doesn't notice. I've been in this business since 1989 and I know what that kind of upkeep costs, and I know nobody sends you a check for it. If what you need is one specific number for one specific gear, his calculator will get you there faster than reading my charts will. Use it. Bookmark it. I do.

What I'm adding here is the part I have that he doesn't: thirty-seven years of watching what actually happens when a family changes a cog, plus the crank and tire math that changes how a gear feels without changing a single tooth.

The eight charts

Every chart below is built around 55. The chainring that produces roughly 55 gear inches on that wheel size sits in the middle of the chart, with lighter gears above it and taller gears below it. Find your ring down the left, your cog across the top, read where they cross.

Each cell gives you two numbers. The big one is gear inches, the number BMXers actually quote. The small grey one underneath is roll-out in inches for one full revolution of the cranks. The hi-vis yellow cells are the 55-inch baseline band, 54.4 to 55.7 gear inches, so you can see at a glance which gears on your wheel size do the same job as a 44/16. Cyan is my own gear: 47/17 on the 20 x 1.75 chart, 36/16 on the cruiser chart.

These are calculated numbers, built on the effective rolling diameter noted above each chart. Your own measured roll-out will land slightly different, because tire pressure and wear both change how tall your wheel sits. Use the charts to compare gears against each other, then measure your bike to get your true figure. Measured is the truth, and there are instructions for doing it further down.

The freestyle chart runs completely different hardware and ends up in the same place. Park bikes use micro-drive, a small ring paired with a very small cog instead of the big ring and mid-size cog a race bike uses, which is why it lists 21 to 27 tooth rings and 8 to 12 tooth cogs. Put a 25/9 and a 44/16 on the same 20-inch wheel and they are within one percent of each other. The numbers on the freestyle chart read a little taller than that, because a 2.3 park tire stands taller than a 1.75 race tire, and the yellow band moves up with it. Same gear, taller wheel.

20 x 1.75 (406) — Standard 20" Race

Effective rolling diameter 20.0". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline. Cyan = Bill's gear.

20 x 1 1/8" (451) — Skinny 20" Race

Effective rolling diameter 20.0". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline.

20 x 1 3/8" (451) — Wide 451 Race

Effective rolling diameter 20.3". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline.

OS20 x 1.65 (451) — Oversize 20"

Effective rolling diameter 21.0". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline.

20 x 2.3 (406) — Freestyle / Park (micro-drive)

Effective rolling diameter 20.6". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline.

24 x 1.75 (507) — Cruiser

Effective rolling diameter 24.0". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline. Cyan = Bill's gear.

26 x 2.125 (559) — Big BMX / Retro Cruiser

Effective rolling diameter 26.2". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline.

29 x 2.3 (622) — 29" Big Bike

Effective rolling diameter 29.1". Big number = gear inches (the number BMXers quote). Small number = roll-out in inches. Yellow = the 55-inch baseline.

Read down any column and look at the size of the steps. On the 20 x 1.75 chart, 44/16 to 45/16 is 1.2 gear inches, about four inches of roll-out, and that one tooth is a real change in gate feel. But 44/16 to 44/15 is 3.7 gear inches, eleven inches of roll-out, which is a completely different bike. That's the habit worth building here: stop thinking in teeth and start thinking in how far you are from 55.

The cruiser chart shows why cruiser ratios sit so much lower than 20-inch ratios and still land in the same neighborhood. The bigger wheel does more of the work. A 20 needs 2.75 to 1 to reach 55 gear inches. A 24 gets there at about 2.31 to 1, which is a 37 on a 16. That is the same relationship as my 36/16 and my 47/17, one wheel size apart.

Crank length and why the same gear feels different

This is the section I get the most arguments about, and most riders have it backwards.

Crank length does not change your roll-out. It changes your leverage.

Look at the formula again. Chainring, cog, wheel diameter, π. Crank length isn't in there, and it isn't in there because it doesn't belong there. Turn the cranks one full revolution and the bike travels the same distance whether those cranks are 165mm or 180mm. What changes is how hard you push to make that revolution happen, and how far your feet travel to do it.

The measure that captures this is gain ratio, and it comes from Sheldon Brown, who spent his life making bicycle math understandable to people who don't like math. Gain ratio is wheel radius divided by crank length, times chainring divided by cog. It tells you how much distance you get for a given amount of foot travel, which describes what your legs are experiencing far better than teeth ever did.

Here's the same 44/16 gear on a 20 x 1.75 wheel, with nothing changed except the cranks.

Both sides of that trade are real and I'm going to give you both.

A 180mm crank gives you 9.1% more torque at the pedal than a 165mm does. That's a genuine mechanical advantage, and it's why the same gear feels easier to push on longer arms. Riders who go up in crank length almost always report their old gear got easier, and they're not imagining it.

The cost is on the other end. That 180mm crank drags your foot around a circle 9.1% longer, every revolution, every lap. At the top of your cadence range, where you're already spinning as fast as your legs will physically cycle, a longer circle means fewer revolutions per minute and less top-end speed for the same gear. Longer cranks favor leverage and taller gears. Shorter cranks favor snap out of the gate and a higher ceiling on cadence. There's no universally correct answer, but there is a mistake I see constantly.

Crank length has to fit the rider's leg before anything else. A crank that doesn't fit costs more than any leverage it gains. It wrecks your position at the top of the stroke, it swings your hip through a range it doesn't want to go, and on a BMX bike it eats the pedal clearance you need for cornering. Kids get put on cranks that are too long constantly, usually because a bike got handed down or a parent bought a size the kid would "grow into." A ten-year-old on 175mm cranks is not making more power. They are pedaling squares and losing the gate.

Get the length right for the rider first. Then, and only then, pick the gear that suits the track. That order matters and it never changes.

If you're shopping, the Speedline Spinners Hollow Alloy Pro Crank is $379.95 and runs 160mm through 180mm, which covers a mini rider right through a pro. The Speedline Elite Carbon Hollow Cranks are $699.95 in 165mm through 180mm. Both live in the Drivetrain collection with everything else on this page.

Tire size changes your gear, and most riders never notice

Wheel diameter is in the formula. Your tire is part of your wheel diameter. So your tire is part of your gear, and it always has been.

A 406 rim has a bead seat diameter of 15.98 inches. The outer diameter of the wheel is that bead seat plus twice the section height of the tire, because the tire sticks up on both sides. Fatter tire, taller wheel, longer roll-out. Skinnier tire, shorter wheel, shorter roll-out. Same chainring, same cog, different gear. Here's what that looks like on a 44/16, changing nothing but rubber.

Swap a 1.75 for a 1.4 and your roll-out shrinks by 3.6%. That is the same effect as taking about a tooth and a half off your chainring, and you did it without touching your drivetrain.

Now think about a wet weekend. It rains Friday night, the track is a mess, and half the pits pulls the fat tires off and bolts on skinny mud tires. Every one of those riders just geared down without knowing it, on top of the traction problem they were actually trying to solve. Some will feel great and never understand why. Some will spin out on the straight and blame the mud. It was the tire. The reverse catches people too: go up from a 1.75 to a 1.85 for a rough national and the gate feels a touch heavier than it did at home.

None of this means you should pick tires based on gearing. Pick tires based on the dirt. Just know that a tire change is a gearing change, and if the gear mattered enough to argue about, the tire matters too. Write down what tire you were on when you write down what gear you were on.

How to actually measure your roll-out

Calculated roll-out gets you close. Measured roll-out is the truth, because your tire is not exactly 20 inches and it squashes under your weight.

1. Find flat ground with fifteen feet of straight run. Garage floor, driveway, parking lot.
2. Put tape on the rear tire sidewall and a matching mark on the ground under it.
3. Mark a starting position on the crank arm.
4. Sit on the bike with your normal riding weight on it and have somebody hold you steady. This matters more than people think, and it's the step everybody skips.
5. Roll forward exactly one full turn of the cranks, back to the same crank position.
6. Mark the ground at the stop and measure between your two ground marks.

One full turn of the cranks, not the wheel. Rolling the wheel one revolution measures the circumference of your tire and tells you nothing at all about your gearing. That's the mistake behind almost every number somebody sends me that can't possibly be right.

The distance you get is your true roll-out. It usually lands a few inches above the chart, because a 20 x 1.75 measures more like 20.2 to 20.6 inches on the outside once it's inflated and on a rim. Measure at the pressure you race. Drop 10 psi and the tire squats, the rolling diameter drops, and the gear gets a touch lighter without you touching anything. Different tire models in the same printed size measure differently too, so if you change brands, measure again.

Picking a gear for your rider and your track

BMX racing is decided in the first thirty feet. Whoever gets out of the gate and into the first turn with clear track ahead wins most of the time, and every gear decision comes back to that.

Newer and smaller riders belong on the lighter side of their range. A gear a rider can spin up quickly builds gate technique and confidence. A gear that's too tall teaches a kid to lug and grind, leaning on strength they don't have yet. You cannot out-gear a bad gate, and a kid who learns to grind out of one will spend years unlearning it.

Strong, developed riders on fast tracks with long straights, big rollers and asphalt corners can carry more gear, because there's real estate out there where top speed actually pays. That's a move from a 44/16 to a 45/16, not from a 44/16 to a 44/15.

Technical, tight or rough tracks reward acceleration out of every corner, so go lighter. You'll spend the whole lap getting back up to speed, and a gear you can pick up quickly beats a top end you never reach. Same logic on hills. An eight-meter pro hill hands you free speed and makes a slightly taller gear livable. On a small local hill you make all your own speed and want a gear that comes off the gate clean.

My advice, one answer instead of a menu: change one thing, then ride it for at least three sessions before you judge it. The first lap on a new gear always feels wrong regardless of whether it is. Riders who swap gears every weekend never give any setup a fair test. If you want a starting point, sit mid-range for your bike size and put the energy into gate practice instead.

How to change your gearing without making a mess of it

Change the cog first

The cog is a small, cheap part and the biggest single lever you have. One tooth on the cog is worth several on the chainring, and there is an exact rule for it that almost nobody writes down: one rear tooth equals your gear ratio, in front teeth. Run a 45/17 and that ratio is 2.65, so one tooth on the cog is worth about 2.65 teeth on the ring. Run a 44/16 and it is 2.75. Run my 47/17 and it is 2.77. Your ratio is the conversion factor, and that is a handy thing to carry in your head at the track. That is why the cog is where you make real changes and the ring is where you fine tune.

If you want it to the decimal, it depends slightly on which way you go, because teeth are whole numbers and the math is not linear. Off a 45/17, dropping to a 16t cog is the same as adding 2.81 teeth up front, and going up to an 18t is the same as taking 2.50 off. Average the two and you land on 2.66.

One rear tooth, converted to front teeth:

Read it as: on a 45/17, one tooth of cog change is worth about 2.65 teeth of chainring. The last column is simply your gear ratio, rounded.

Our Speedline CNC Machined Cro-mo Rear BMX Race Cogs are $29.95 in 14t, 15t, 16t, 17t and 18t. A 15 and a 17 alongside the 16 you run gives you three usable gears out of one drivetrain for under sixty bucks, and it all fits in a tool bag.

Change the chainring to fine tune

One tooth on the ring is a smaller step, worth about 1.25 gear inches, which makes it the right tool when you're already close and want a nudge rather than a jump. Rings cost more and are heavier to haul to the track, so most riders end up keeping one ring and a small stack of cogs. That's the setup I'd build from scratch.

Speedline chainrings land in September, shipping to us on September 9th alongside the new Speedline cranks. If you are building a gear plan for next season, that is the piece to wait for. In the meantime the cogs and the chain below are in stock and will get you most of the way there.

Watch your chain length

This is where gear changes go sideways in the pits. Take a tooth off the cog and the chain is suddenly a little long. Add one and it may be short. A standard chain only adjusts a full link at a time, which is too coarse a step, and you end up with the axle slammed to one end of the dropout just to get tension. The KMC HL1L Wide Half-Link Chain at $34.95 adjusts in half-link increments, so the chain lands right and the wheel sits where you want it. If you plan to touch your gearing at all this season, buy it up front.

Use tensioners

Once the wheel is where you want it, tensioners hold it there through gate slams and keep the rear end square. We stock both common axle sizes: the Speedline CNC'd Alloy Chain Tensioner Kit in 3/8" (10mm) at $24.95 and the 15mm version at $29.95.

Change one thing at a time

Change cranks and gearing in the same week and you will never know which one did what. Do the cranks, ride it, then do the gear.

What the rulebook actually says

I went through the 2025 USA BMX / BMX Canada rulebook before writing this, specifically Section X.B, "Equipment (Bicycle)." It lists sixteen numbered equipment rules and not one of them restricts gearing, gear ratio, chainring size, cog size, or roll-out. I searched the full document for every one of those terms and came up empty.

So as I write this in July 2026, USA BMX does not appear to limit what gear you run in any class. I'm not going to print a roll-out limit number, because there isn't one in the book I read, and a stale number in a blog post is exactly what gets a family turned away at tech inspection. Check the current rulebook yourself before your race. It's free, at usabmx.com/riders/rulebook. Rules change, and your track operator has final say at their track.

Some overseas federations do restrict roll-out for youth categories in other cycling disciplines, which is why gear-restriction charts turn up on British and European club sites. Those numbers don't apply to a USA BMX race.

What I'd tell you to do

Measure what you're on now, cranks and tire included. You can't tune what you haven't measured, and half the riders I talk to have no idea what their real roll-out is.

Buy a half-link chain and one cog on either side of the one you run. That's about $95 and it gets you three real gears without a trip to the shop.

Check the crank length against the rider before you touch a tooth. If the cranks are wrong for the leg, nothing you do with gearing fixes what that's costing.

If you race both a 20 and a cruiser, use the charts to get the two bikes inside about four inches of roll-out of each other. Gate timing carries across, cadence carries across, and you stop re-learning a bike every time you switch. Mine sit 4.1 inches apart and my legs never knew the difference.

Then leave it alone and go ride. I've watched a lot of people spend a season chasing teeth when they should have been running motos.

Keep reading

• BMX Frame Sizing Chart — get the frame right first.
• The Complete BMX Race Bike Buyer's Guide
• Keep Your BMX Bike Clean, Tight and Ready to Ride
• The Great Debate: BMX Gearing — the same subject in gear inches.
• Shane Jenkins' gear calculator at bmxultra.com — the most complete BMX gear chart anywhere.
• BMX Track Locator — go test what you just set up.

Frequently asked questions

What is roll-out in BMX?

Roll-out is how far your bike travels in a straight line for one complete revolution of the pedals. It's the single number that tells you how hard or easy a gear is to push, and it lets you compare gears across different chainring, cog and wheel combinations.

How do I calculate BMX roll-out?

Roll-out in inches equals the chainring teeth divided by the cog teeth, multiplied by the wheel diameter in inches, multiplied by 3.1416. A 44/16 on a 20-inch wheel is 44 ÷ 16 = 2.75, times 20 = 55, times 3.1416 = 172.8 inches.

Is roll-out the same thing as gear inches?

They describe the same gear two different ways. Gear inches is the ratio times the wheel diameter, which gives you the familiar number around 55 that American racers quote. True roll-out multiplies that by π (3.1416) to get the actual distance rolled, around 173 inches for the same gear. Multiply gear inches by 3.1416 to convert, and divide by 3.1416 to go back.

What does "55 inches" mean in BMX gearing?

It is gear inches: chainring divided by cog, multiplied by the wheel diameter. A 44/16 on a 20-inch wheel is 2.75 times 20, which is exactly 55.00 gear inches. That is the BMX race standard and it is what a racer means when he says he runs 55. It is not the distance the bike travels — multiply by 3.1416 for that, and the same gear rolls out 172.8 inches. Every chart on this page is centered on 55 so you can tell immediately whether a gear is taller or lighter than the standard.

Why are freestyle 25/9 and race 44/16 the same gear?

Because gearing is a ratio, not a tooth count. A 44/16 is 2.7500 to 1 and a 25/9 is 2.7778 to 1. On the same 20-inch wheel that is 55.00 gear inches against 55.56, a difference of 0.56 gear inches or 1.01%. Racing and freestyle arrived at those numbers separately and for different reasons — freestyle runs tiny sprockets so the drivetrain survives grinds and hard landings. The parts look nothing alike and the gear is the same. A park bike's taller 2.3 tire does raise the effective gear a little, which is why the freestyle chart on this page reads slightly higher.

Does crank length change my roll-out?

No. Crank length changes your leverage, not your roll-out. A 44/16 on a 20-inch wheel rolls 172.8 inches per crank revolution whether you run 165mm or 180mm cranks. What changes is effort and foot travel: a 180mm crank gives about 9.1% more torque at the pedal than a 165mm, but the pedal circle is about 9.1% longer, which costs you top-end cadence. Longer cranks favor leverage and taller gears, shorter cranks favor spin-up out of the gate, and the length has to fit the rider's leg before either consideration matters.

What is gain ratio?

Gain ratio is wheel radius divided by crank length, multiplied by chainring teeth divided by cog teeth. It comes from Sheldon Brown and it measures how much distance you get for a given amount of foot travel, which describes what your legs actually feel better than gear inches does. A 44/16 on a 20-inch wheel with 165mm cranks has a gain ratio of 4.233. The same gear on 180mm cranks is 3.881.

Does tire size change my gearing?

Yes. Wheel diameter is part of the roll-out formula and your tire is part of your wheel diameter. On a 406 rim with a 44/16, a 20 x 1.4 rolls out 3.59% shorter than a 20 x 1.75, which is roughly the same effect as dropping a tooth and a half off the chainring. Riders who swap to a skinny mud tire for a wet track change their gearing at the same time, usually without realizing it.

Does USA BMX limit gearing or roll-out?

The 2025 USA BMX / BMX Canada rulebook's Equipment (Bicycle) section does not contain any gearing, gear ratio, or roll-out restriction. It does restrict pedals for riders 12 and under and for Novice riders of any age, and it sets maximum and minimum wheel-and-tire diameters for the 20-inch and cruiser classes. Rules change, so check the current rulebook at usabmx.com before you race, and remember the track operator has final say on equipment at their track.

What gear should a beginner run on a 20-inch bike?

Start on the lighter side. A 44/16 is the long-standing default on a pro-size 20 and a good place to begin, and smaller riders on micro, mini and junior bikes should be lighter still. A gear you can spin up quickly teaches good gate technique. Build the rider first, then add gear.

How do I match my cruiser gearing to my 20-inch bike?

Calculate the roll-out of your 20-inch gear, then pick the cruiser gear closest to it. A 47/17 on a 20 rolls out 173.7 inches and a 36/16 on a 24 rolls out 169.6 inches, a difference of 4.1 inches or 2.34%, which is why those two gears feel the same underneath you even though the numbers look unrelated. Get your two bikes inside about four inches of each other and they'll feel like the same bike.

Why is freestyle gearing so different from race gearing?

Freestyle and park bikes run micro-drive, which pairs a small chainring of roughly 23 to 30 teeth with a very small cog of 8 to 12 teeth. The ratio ends up similar to a race gear, but the parts are half the size, because a small ring tucked up out of the way survives grinds and hard landings in a way a 44-tooth race ring never would. Read freestyle gears off a freestyle chart, not a race chart.

Should I change the cog or the chainring?

Change the cog when you want a real change and the chainring when you want a small one. One tooth on the cog moves your gear roughly twice as much as one tooth on the chainring. Cogs are also cheaper and easier to carry, so most racers keep one chainring and a few cogs.

Do I need a new chain when I change my gearing?

Often, yes, or at least a different chain length. Changing cog size changes how much chain you need, and a standard chain only adjusts in full links, which is frequently too big a step. A half-link chain adjusts in half-link increments and lets you set the chain correctly while keeping the rear wheel where you want it in the dropout.

Why do calculated and measured roll-out numbers differ?

Gear charts are calculated from a nominal wheel diameter, while your bike has a real tire on it that sits a little taller than nominal and squats under your weight. Tire pressure, tire wear and tire model all move the number. Use a chart to compare gears against each other, then measure your own bike at race pressure with your weight on it to get your true figure.