BMX Gear Ratio Chart: Choosing Your Race Gearing

• The short answer
• Ratio, gear inches, rollout: three ways of saying the same thing
• Real wheel diameter, the figure everyone forgets
• The BMX gear ratio chart
• Why gearing matters more in BMX Race than anywhere else
• Short or tall: reading the track
• Typical ranges by frame size
• Chainring, sprocket or rear cog: where to make the change
• What a gear change does to the rest of the bike
• Measuring your own rollout
• The gearing mistakes we put right in the workshop
• Frequently asked questions

The short answer

• The sum. Front teeth ÷ rear teeth. The quotient is the gear ratio. A 44x16 gives 2.75.
• What that is worth. 55.0 gear inches, and a rollout of 4.39 m per crank revolution, on a 451 wheel fitted with a 20 x 1-1/8".
• The useful window. About 2.30 for a young rider, 3.00 for a Pro.

You adjust one tooth at a time, almost always at the rear, and you confirm it on the track rather than on paper.

Figures for a 451 wheel fitted with a 20 x 1-1/8" tyre. The full chart, 41x18 to 45x15, is further down.

Ratio, gear inches, rollout: three ways of saying the same thing

Nobody at the side of the track is quite talking about the same thing. One rider quotes a ratio, the next their gear inches, the third their rollout. It is the same setting seen at three levels of precision.

The gear ratio

ratio = front teeth ÷ rear teeth. 44 ÷ 16 = 2.75. It is the easiest number to swap with another rider, and the only one that depends purely on the drivetrain. Its weakness is that it ignores the wheel. Two bikes both running 44x16, one on 406 rims and one on 451, do not cover the same ground.

Gear inches

gear inches = ratio × the diameter of the built wheel, in inches. That diameter is the wheel with the tyre on it, not the bare rim. On a wheel measuring 20.0 inches over the tyre, a 44x16 gives 2.75 × 20.006 = 55.0 gear inches. This is the unit American and British riders quote first. Mind the wording, though. Gear inches are not a distance travelled but an equivalent wheel diameter, inherited from the penny-farthing, so nobody should ever write "55 inches of rollout".

Rollout

rollout = ratio × wheel circumference = ratio × π × diameter. On that same wheel the circumference is π × 508.15 mm = 1,596 mm, so a 44x16 gives 2.75 × 1.596 = 4.39 m per crank revolution. This one really is a distance, in metres, and it is the only one of the three you can check with a tape measure in the pits — which is why it settles arguments. American riders often quote rollout in inches instead: 4.39 m is 173 inches, or 14 ft 5 in. Note that this is not the same number as gear inches, even though both are quoted in inches.

Real wheel diameter, the figure everyone forgets

"Twenty inch" is a label, not a measurement: two BMX Race wheels both called 20" share neither the same rim diameter nor the same overall diameter once a tyre is fitted. This is the single most common calculation error on the subject.

The reliable method starts from the standardised diameter of the rim, its ETRTO figure — the European tyre and rim sizing standard, printed on the sidewall of the tyre as 406, 451 or 507. To that you add twice the tyre width, since the tyre stacks its own height at the top and at the bottom of the wheel:

developed diameter = ETRTO rim diameter + (2 × tyre width)

An inch of width converts at 25.4 mm, so a 20 x 1-1/8" measures 1.125 × 25.4 = 28.575 mm. A 451 rim fitted with that tyre gives 451 + 57.15 = 508.15 mm, or 20.01 inches — exactly the nominal figure, which is what makes this build the natural benchmark for gearing charts. Everything else drifts away from it, and by more than people expect.

Read it this way. Moving from a 451 in 1-1/8" to a 406 in 1.75 without touching a single tooth shortens the gear from 55.0 to 53.6 gear inches, roughly half a rear tooth. Going to a 24" Cruiser on the same tooth counts stretches it from 55.0 to 64.5, or +17.3% — about two and a half rear teeth. Your choice of BMX tyres is therefore part of your gearing, even when you never meant it to be.

One honest caveat on these figures. They use nominal widths, while the fitted width of a mounted tyre depends on internal rim width and on pressure. More to the point, the "rim + 2 × width" formula overestimates the rolling diameter by 2 to 3%, because a tyre is not as tall as it is wide and it squashes under the rider. None of that changes which tooth count to order, but keep it in mind when you compare the calculation against a ground measurement.

The BMX gear ratio chart

Every common combination, from 41 to 45 teeth at the front and 15 to 18 at the rear, on a wheel measuring 20.01 inches over the tyre (451 rim, 20 x 1-1/8"). For any other build, multiply the ratio by your own diameter. Three benchmarks before you read it:

• 44x16 = 55.0 gear inches, the figure everybody works from.
• Below 50, you are looking at a Micro or Mini build.
• Above 57, you are looking at a Pro build.

Two very different tooth pairings sometimes land in the same place — a 41x15 (2.73) and a 44x16 (2.75) are the same gear to within 0.6%. What separates them is weight, chain length, ground clearance and wear. A small rear cog wears faster than a large one, because the same load passes through fewer teeth in mesh.

Why gearing matters more in BMX Race than anywhere else

On a geared bike, a poor chainring choice is corrected by shifting. Here there is nothing to correct. A moto lasts about thirty-five seconds and most of the result is decided in the first five: between the gate dropping and the bottom of the hill, a rider who takes half a wheel usually keeps it to the finish, because the track is narrow and passing is expensive. If the shape of a race day is still new to you, our guide on how to start BMX racing sets the scene.

The start is about the harshest load a BMX ever sees. The rider is out of the saddle, weight forward, cadence close to zero, fighting the inertia of the bike. Too tall a gear gives a soft first pedal stroke and costs you time out of the gate. Too short and the rider hits maximum useful cadence before the end of the first straight, spinning helplessly while everyone else is still accelerating. It is the only setting that acts on both ends of the race. Bar width changes your position; gearing changes the clock.

Short or tall: reading the track

The trade-off fits in one line: shorter = acceleration, taller = top speed. Everything else is deciding which of the two a given track rewards.

The start hill

A 5 m hill is shallower and hands back less free speed. The rider generates the acceleration, and a short gear helps. An 8 m hill gives a lot of speed away the moment the gate drops; the rider arrives at the bottom already flying, and a short gear spins out too early. On an 8 m start, the tendency is to go one tooth taller.

Straight length

A short, technical track where you have to get back up to speed out of every turn calls for a short gear, because you are picking up from low speed a dozen times a lap. A fast track with a long first straight rewards top speed, and therefore a taller gear.

The rider

A light, fast-spinning rider carries a taller gear than a heavier, more explosive rider on the same frame size — the latter is better off shortening it to use that power from the first pedal stroke. That power still has to go through the bike. A handlebar of the right width lets you pull against it out of the gate, where a bar that is too wide opens the shoulders and kills the leverage.

Either way, never change two variables at once. One tooth up or down, then the same timed section repeated several times in the same conditions. Plenty of riders keep a spare rear cog in the kit bag to adjust on the spot.

Typical ranges by frame size

Micro, Mini, Junior, Expert and Pro are frame sizes, not race classes. These are ballpark figures, not rules. They assume a 451 wheel fitted with a 20 x 1-1/8", and riders sit outside their range all the time without it being a mistake. To settle the frame size itself, see our BMX size chart.

Cruiser has to be worked out from scratch. With a 507 rim and a 1.75 tyre, the wheel measures 23.46 inches instead of 20.01, so a 44x16 no longer gives 55.0 but 64.5 gear inches, or 5.15 m per crank revolution. Carrying the tooth counts over from a 20" bike when you move to a BMX Cruiser therefore stretches the gear by 17.3%, the equivalent of about two and a half rear teeth. A cog on its own will not cover it, since you would need 19 teeth and nobody makes one. You correct at both ends — on that wheel, a 42x18 gives 54.7 gear inches and a 43x18 gives 56.0.

Crank arm length interacts with gearing too. Shorter arms spin the legs faster for the same effort and make a tall gear more tolerable. On a young rider, going from 165 to 155 mm often changes the gate start more than a tooth does.

Chainring, sprocket or rear cog: where to make the change

A rear tooth is not worth a front tooth, and the gap is wider than most riders assume. Starting from a 44x16:

• +1 tooth at the front (45x16): ratio 2.81, or +2.3%.
• −1 tooth at the rear (44x15): ratio 2.93, or +6.7%.
• +1 tooth at the rear (44x17): ratio 2.59, or −5.9%.

One rear tooth is therefore worth roughly three front teeth: you would have to run 47x16 (2.94) to match a 44x15 (2.93). Hence the standard practice — you adjust from the rear, with a part that is small, cheap and swappable in the pits — and you keep front tooth changes for fine tuning.

Then there is the question of what you have up front. A BMX chainring is a toothed ring bolted to a spider: you replace it on its own, four or five bolts, cranks left in place. A one-piece BMX sprocket is machined from a single piece, teeth and spider together — stiffer, less hardware, but the whole thing has to be replaced. If you vary front tooth count through the season, a chainring on a spider stays the practical answer; if you adjust from the rear, a one-piece sprocket costs you nothing.

What a gear change does to the rest of the bike

Chain length and wheel position

With the BMX chain length unchanged, one tooth more or less moves the rear axle forward or back in the dropouts by about 3 mm. On a 44x16 set at 380 mm of chainstay length, going to 45x16 brings the axle to 376.5 mm, and going to 44x15 pushes it out to 382.9 mm. Those three millimetres sit comfortably inside the adjustment range. A three-tooth swing at the front — 41x16 instead of 44x16 — asks for more than 10 mm of travel and usually runs the wheel past the end of the dropout.

A conventional chain only adjusts in 25.4 mm steps, because inner and outer links alternate. A half-link chain, with every link identical, halves that to 12.7 mm and rescues a build that falls between two chain lengths. Keep one spare.

Tension and alignment

After any change, aim for about a centimetre of vertical play at the middle of the top run. Too tight and the chain drags and destroys hub bearings; too loose and it drops off coming off a jump. On horizontal dropouts, a chain tensioner locks the setting. Check your chainline as well: the front teeth must sit in the same plane as the rear cog, and you correct that with bottom bracket spacers, never by forcing the chain.

Ground clearance

A big front ring hangs lower. On a frame with a low bottom bracket, or with 180 mm crank arms, a 45 or 47 tooth chainring eventually grounds out in a berm and on landings. Check its clearance along the chainstay too.

Freewheel, cassette driver, spline drive or bolt drive

The tooth counts available depend on your hardware. At the rear, a screw-on freewheel carries its own pawl mechanism, comes in 15 to 18 teeth and needs the correct removal tool. A hub with a cassette driver takes a splined cog held on by a lockring: lighter, cheaper to renew and available in small sizes — the standard race setup, because it makes the change quick.

At the front, everything depends on the crankset. On freestyle cranks the one-piece sprocket mounts either spline drive, pressed onto the splines of a 19, 22 or 24 mm crank spindle, or bolt drive, bolted straight to the crank arm. In race, the chainring bolts to a spider, and there are two bolt circles that do not mix: 4-bolt at 104 mm and 5-bolt at 110 mm.

This is where the manufacturer decides your build, and the spec sheets say so in black and white. Elevn lists its Flow chainring in 4-bolt 104 mm, 36 to 50 teeth, in 4.4 mm 6061-T6. Rennen covers the same 4-bolt 104 mm from 32 to 52 teeth in 7075-T6, with a threaded version that saves you the chainring nut wrench. Tangent stays on 5-bolt 110 mm, 36 to 48 teeth. So count the bolts and measure the circle on your spider before you order — that is a comparison a single-brand catalogue cannot make for you. Finally, front teeth, rear teeth and chain all have to share the same format, 1/8" or 3/32".

Measuring your own rollout

The calculation gives you a theoretical figure; the ground measurement gives you the real one, with the tyre at pressure and the rider on the bike.

1. Inflate the tyre to race pressure — the pressure you will actually ride.
2. On flat ground, set one crank arm strictly horizontal and pointing forward, and mark the ground directly below the bottom bracket spindle.
3. Sit on the bike, in position, and roll forward exactly five crank revolutions, smoothly, coming back to the same crank position.
4. Mark the ground again at the same point, measure with a tape, divide by five.

Five revolutions rather than one, because the marking error of a centimetre or so is then divided by five. As a sanity check, on a 451 wheel in 1-1/8", five revolutions come to roughly 20.45 m in 41x16, 21.95 m in 44x16 and 23.41 m in 44x15.

More than 5% out and you have an under-inflated tyre, a different rim diameter from the one you assumed, or a miscount. Why 5% and not 3%? Because the "rim + 2 × width" formula already overestimates the rolling diameter by 2 to 3%. A correct measurement therefore comes out slightly under the theoretical figure, and a tighter threshold would sound the alarm on a perfectly good bike. Where the two disagree, the measurement wins.

Write the measured figure down with the date, the track and how the gate start felt. After a season, that notebook is worth more than any chart.

The gearing mistakes we put right in the workshop

• Comparing ratios across different wheels. "I run 44x16" means nothing until you know whether that is on 406, 451 or 507: the spread reaches 12 gear inches.
• Changing tyre and assuming nothing moved. From a 20x1.50 to a 20x1.75 on the same 406 rim, the gear goes from 52.2 to 53.6 gear inches with identical tooth counts.
• Going taller because someone came past at the bottom of the hill. More often the problem is positioning or the first pedal stroke. Film your start before you order a part.
• Changing the chainring when a rear cog would have done. Three teeth at the front for the effect of one at the rear: more expensive, and the wheel moves far more in the dropouts.
• Mixing tooth formats. A 1/8" chainring with a 3/32" chain, or the reverse, and the drivetrain skips on the first hard pedal stroke.
• Fitting a Pro gear to an Expert rider. The gearing of the best riders is calibrated for a power output and a cadence that are not yours.
• Setting chain tension before cutting the chain. Position the wheel in the middle of the dropout range first, then cut.

One last reflex. When rear teeth start to hook or the chain skips under load, that is no longer a gearing problem but a wear problem. Replace the chain and the rear cog together.

To change your gear, most of what you need is in our rear cogs and our chainrings, and the rest in the BMX Race drivetrain aisle. As a multi-brand shop, we compare standards instead of pushing one range: tell us your rim diameter, the tyre width fitted, your rear hub type and your current gearing, and we confirm which tooth counts will actually mount before we dispatch, usually within 24 to 72 hours. You can spread the cost over 2 or 3 payments.

Frequently asked questions

What is a BMX gear ratio?

It is the single ratio of the drivetrain, found by dividing the front tooth count by the rear tooth count. A 44x16 gives 2.75, so the wheel turns 2.75 times per crank revolution. Multiply that ratio by the diameter of the built wheel and you get gear inches; multiply it by the circumference and you get rollout, a distance in metres.

How many teeth should a BMX Race bike have?

At the front, the useful range runs from about 39 to 47 teeth, with 43, 44 and 45 the most common. At the rear, 15 to 18 teeth, with 16 the usual starting point. What matters is not the tooth count but the pair: a 41x15 and a 44x16 give practically the same gear.

What is 44/16 gearing in gear inches?

A 44x16 gives a ratio of 2.75. On a wheel measuring 20 inches over the tyre (451 rim with a 20 x 1-1/8"), that is 55.0 gear inches and a rollout of 4.39 m per crank revolution. It is the benchmark gear for Junior and Expert frames, and the logical place to start testing a tooth up or a tooth down.

How do you calculate BMX rollout?

Multiply the ratio by the wheel circumference: rollout = (front teeth ÷ rear teeth) × π × developed diameter. Developed diameter is the ETRTO rim diameter — the 406, 451 or 507 printed on the tyre sidewall — plus twice the tyre width. For the real-world figure, measure the distance covered on the ground over five crank revolutions and divide by five.

Should I change the chainring or the rear cog?

The rear cog, in the vast majority of cases. One rear tooth is worth about three front teeth: from a 44x16, going to 44x15 stretches the gear by 6.7% where a 45x16 only stretches it by 2.3%. The rear cog is also smaller, cheaper and swappable in the pits between motos.

Does a smaller rear cog accelerate faster?

No, it is the other way round. A smaller cog makes the gear taller — from 44x16 to 44x15 the ratio goes from 2.75 to 2.93, or +6.7% — so each crank revolution covers more ground, but it takes more force to get the first pedal stroke moving. To accelerate harder out of the gate you go up in teeth at the rear instead, from 16 to 17 for example.

How do you change a BMX cog or sprocket?

It depends on the fitting. A screw-on freewheel comes off with the matching removal tool and a spanner; a cog on a cassette driver comes off once you undo its lockring. At the front, a chainring on a spider unbolts with an Allen key and a chainring nut wrench, cranks left in place, while a one-piece sprocket means pulling the crank arm. Always check the bolt circle and the chain format before ordering from our rear cogs or our sprockets.

What gearing for a 24-inch Cruiser?

Recalculate rather than copy. A 507 rim with a 24x1.75 measures 23.46 inches against 20.01 for a 451 wheel, so on identical tooth counts the gear climbs from 55.0 to 64.5 gear inches, or +17.3% — about two and a half rear teeth. A cog alone will not do it, since you would need 19 teeth. Correct at both ends: a 42x18 gives 54.7 gear inches.

What gear ratio for street or park?

Nothing like a race gear. Street and park run micro gearing such as 25x9 or 28x9, chosen for ground clearance and weight rather than for rollout. A 25x9 works out at 2.78, practically the same ratio as a race 44x16 (2.75) — the difference is the hardware, not the number. Those tooth counts need a short cassette driver and a 1/8" chain, and they will not mount on a BMX Race drivetrain.

Does a taller gear make you faster?

Only at the end of a straight, and only if you have the power to pull it. A taller gear raises the speed reached at a given cadence, but blunts acceleration off the gate, where most of a thirty-five second moto is decided. On a track where you have to get back up to speed out of every turn, a shorter gear is often faster on the clock.