Dirt Bike Gearing Explained: Sprockets, Ratios And Speed Per Gear

Three Reductions Between The Crank And The Tire

A motocross engine makes its power at high rpm, and the rear wheel turns far slower. Three gear stages sit between them, one after another. The crankshaft gear drives the clutch basket gear (the primary drive). The clutch turns the gearbox main shaft, and the selected gear pair turns the countershaft. The countershaft sprocket then pulls the chain around the rear sprocket (the final drive).

Each stage is a ratio, and the overall ratio is the three multiplied together. KTM prints all three for the 2024 250 SX-F. The primary drive is 24:72, which is 3 to 1. First gear is 13:32 (2.46), fifth is 21:25 (1.19). The US bike runs a 14 tooth front sprocket and a 51 tooth rear, 3.64 to 1. Press the steps to run each stage.

Multiply it out and fifth gear is 3.000 x 1.190 x 3.643, about 13.0 crank turns per wheel turn. First is 3.000 x 2.462 x 3.643, about 26.9. Only the last stage is easy to change at home, and it changes every gear at once.

Final Drive Ratio: Rear Teeth Divided By Front

The final drive ratio is the rear sprocket tooth count divided by the front. 51 / 14 = 3.64, so the countershaft turns 3.64 times for each turn of the rear wheel. Most sprockets have the tooth count stamped on the face, so you can read it without counting.

A bigger number means shorter (lower) gearing. The engine spins faster at a given road speed, so the bike pulls harder out of corners and reaches the top of each gear sooner. A smaller number is taller gearing, with more speed per gear and a softer pull. Different combinations can land in almost the same place, for example 14/48 = 3.43, 13/45 = 3.46 and 15/52 = 3.47.

Why One Front Tooth Equals About 3.6 Rear Teeth

You'll often hear riders say one tooth up front is worth about three at the back. Here's the math. Going from a 14 to a 13 tooth front sprocket on a 51 rear takes the ratio from 3.643 to 3.923, which is 7.7 percent shorter. Going from a 51 to a 52 tooth rear with the 14 front takes it from 3.643 to 3.714, only 2.0 percent.

So the front tooth does 7.7 / 2.0, or roughly 3.8 times as much. The general version: one tooth changes the ratio by about 1 divided by that sprocket's tooth count. A front tooth is about 1/14 and a rear tooth about 1/51, so one front tooth equals about 51/14 rear teeth. That is the final drive ratio itself. On modern MX bikes running 3.5 to 3.9 to 1, "three" undersells it a little.

That is why riders use the front sprocket for big changes and the rear for fine tuning. Try it in the swap lab. The readouts compute ratio, the change against the stock 14:51, speed in the gear you pick, and how far the axle has to move on the same chain.

Want it for your own bike with your own gear ratios? Use the dirt bike gearing calculator.

Speed Per Gear: Worked Example

Road speed per 1,000 rpm equals tire circumference divided by the overall ratio. A 110/90-19 tire is nominally 19 x 25.4 + 2 x (110 x 0.90) = about 681 mm tall, about 2.14 m around. One thousand crank turns in fifth on the KTM 250 SX-F is 1,000 / 13.0 = 77 wheel turns, so 165 m per 1,000 revs, or 9.9 km/h (6.1 mph). These figures are our arithmetic from the manual's ratios and a nominal tire, so treat them as close, not exact.

The 250 four-strokes rev much higher than the 450s, so they carry about 40 percent more overall reduction to reach similar speeds.

OEM Gearing By Model

These are the numbers printed in current manuals. KTM lists tooth counts and Honda lists decimal ratios. Yamaha gives both.

KTM writes gears as driving:driven teeth, so 13:32 is a 2.46 reduction and 22:21 is slightly faster than one to one. Honda's tooth counts in brackets are our inference from its printed ratios. KTM lists 48, 50, 51 and 52 tooth rear sprockets for the 2024 250 SX-F.

Taller Or Shorter: Which Way To Go

• Shorter (bigger rear or smaller front). Higher numerical ratio. Quicker pull, less speed per gear. Useful on tight tracks where you live between second and third, or on a bike that lugs out of turns.
• Taller (smaller rear or bigger front). Lower ratio. More speed per gear and fewer shifts. Desert racers gear very tall; Transmoto reported CRF450s at the Finke Desert Race on 15/44 or 14/42 against a stock-style 13/48.
• Honda Torque Control. Honda's CRF250R and CRF450R manuals warn that the traction control is calibrated for the recommended tires and sprockets, so non-stock sprockets can change how it behaves.

Gearing also changes chain angle, which changes how the rear end squats or lifts under power. Cycle World's Kevin Cameron explains that the chain pull has a component that pushes on the swingarm, so a sprocket swap can make a bike feel different over bumps on the gas, not only faster or slower.

After A Sprocket Swap: Axle, Slack And Torque

A bigger rear sprocket needs more chain to wrap it. On the same chain, the axle has to move forward. Each extra rear tooth needs about half a pitch more wrap, so the axle moves forward roughly 4 mm on a 520 chain (our geometry calculation, the same model the swap lab uses). KTM's chain adjusters have 32 mm of range, and the manual says that range exists so different secondary ratios work with the same chain length. Outside the range, you add or remove links.

1. Fit the new sprocket and torque its nuts to the manual figure. Yamaha's 2026 YZ250F lists 42 N·m for the rear wheel sprocket nuts. Transmoto notes self-locking sprocket nuts wear out after two or three tightenings, so replace them.
2. Set slack with the manual's method. KTM is 58 to 61 mm, measured at the end of the chain slider on a stand.
3. Match the adjuster marks on both sides so the wheel stays aligned.
4. Torque the axle nut. KTM's M22 nut is 80 Nm (59 lbf ft).
5. Recheck slack after the first ride.

Fitting a new rear sprocket on a worn chain is a false saving. KTM says the chain and both sprockets should always be replaced together. Our chain slack and wear guide covers how to measure a worn chain.

Common Gearing Mistakes

• Treating every tooth the same. One front tooth moves the ratio about 7 to 8 percent. One rear tooth moves it about 2.
• Judging gearing on a worn drivetrain. A stretched chain and hooked sprockets waste power and feel rough. Check wear first.
• Forgetting the slack. Any sprocket change moves the axle, so the old slack setting is gone.
• Changing the front when you wanted a small step. If one rear tooth is enough, leave the front alone.
• Not writing it down. A month later nobody remembers whether the 50 or the 52 went on.

Track It In MotoMind

MotoMind keeps a mod log per bike, so your sprocket combination, the date and the cost live in one place. The app tracks maintenance by engine hours and shows tasks like chain inspection and chain and sprocket replacement as upcoming, due or overdue. Its GPS ride tracker adds ride time to the bike's hours.

FAQ

How do you calculate dirt bike gearing?

Divide the rear sprocket teeth by the front sprocket teeth. A 14 tooth front and 51 tooth rear is 51 / 14 = 3.64. For the overall ratio in a gear, multiply the primary ratio, that gear ratio and the final drive. On a KTM 250 SX-F in fifth that is 3.000 x 1.190 x 3.643, about 13.0.

Is one tooth on the front sprocket equal to three on the rear?

Roughly, and usually a bit more. One tooth changes the ratio by about 1 divided by that sprocket’s tooth count, so one front tooth equals about rear teeth divided by front teeth. On a 14:51 setup that is about 3.6 rear teeth.

Does a bigger rear sprocket make a dirt bike faster?

It makes it accelerate harder and lowers the speed in each gear. A bigger rear sprocket is shorter gearing. A smaller rear or bigger front sprocket is taller gearing with more top speed per gear.

What is stock gearing on a KTM 250 SX-F?

The 2024 US model runs 14:51, the EU model 14:52 and the XC-F US 13:52, per KTM’s owner’s manual. The 2024 450 SX-F runs 13:51 and the 250 and 300 SX two-strokes run 14:49.

Do I need a new chain when I change sprockets?

If the chain is worn, yes. KTM says the chain and both sprockets should be replaced together because a new sprocket wears fast on an old chain. If the chain is in spec, the adjusters may cover a small change; KTM’s give 32 mm of range.

How fast does my dirt bike go per 1,000 rpm?

Divide tire circumference by the overall ratio. With a nominal 110/90-19 tire, about 2.14 m around, a KTM 250 SX-F in fifth makes about 9.9 km/h (6.1 mph) per 1,000 rpm. Our gearing calculator does it for any setup.

Should I change the front or rear sprocket?

Use the front for big changes, about 7 to 8 percent per tooth, and the rear for small ones, about 2 percent per tooth on a 50 tooth sprocket. Many riders prefer rear changes because a smaller front sprocket also wraps the chain around a tighter radius.