Free Tool
Dirt Bike Gearing Calculator
Enter your front and rear sprocket teeth. The calculator gives the final drive ratio, how much shorter or taller it is than the setup you compare against, and, when the gearbox ratios are known, road speed in every gear. Pick a bike to load the ratios printed in its owner's manual.
Gearing Calculator
| Gear | Overall ratio | Speed now | Compare setup |
|---|
Speeds use nominal tire size and ignore tire growth and slip, so treat them as close, not exact. After any sprocket change, reset chain slack and torque the axle nut to your manual's spec.
How The Gearing Calculator Works
The final drive ratio is rear teeth divided by front teeth. The percent change compares it with the "compare to" sprockets you enter, so +7.7% means 7.7 percent shorter gearing (more pull, less speed per gear). The "rear teeth per front tooth" number is the final drive ratio itself, because one tooth changes the ratio by about one over that sprocket's tooth count.
For speed, the calculator multiplies primary ratio x gear ratio x final drive to get engine turns per wheel turn, then divides rpm by that and multiplies by tire circumference. The bike presets load the primary and gearbox ratios printed in each owner's manual. The full explanation, with animated figures, is in dirt bike gearing explained.
Where The Preset Numbers Come From
| Bike | Primary | Gears 1st to 5th | Stock sprockets | Source |
|---|---|---|---|---|
| KTM 250 SX-F US 2024 | 24:72 | 13:32, 16:32, 17:28, 19:26, 21:25 | 14:51 | Owner's manual, PDF p. 149, 153 |
| KTM 450 SX-F 2024 | 29:72 | 14:28, 16:26, 18:24, 21:24, 22:21 | 13:51 | Owner's manual, PDF p. 154, 157 |
| KTM 250 / 300 SX 2024 | 26:72 | 14:28, 16:26, 18:24, 21:24, 22:21 | 14:49 | Owner's manual, PDF p. 132, 135 |
| Honda CRF250R 2025 | 3.047 | 2.384, 1.933, 1.600, 1.350, 1.153 | 3.846 (13:50) | Owner's manual, PDF p. 80 |
| Honda CRF450R 2025 | 2.357 | 2.133, 1.705, 1.421, 1.210, 1.043 | 3.769 (13:49) | Owner's manual, PDF p. 79 |
| Yamaha YZ250F 2026 | 3.353 | 2.143, 1.750, 1.444, 1.222, 1.042 | 13:50 | Owner's service manual, PDF p. 56 to 57 |
| Yamaha YZ450F 2026 | 2.481 | 2.000, 1.625, 1.350, 1.136, 1.000 | 13:50 | Owner's service manual, PDF p. 55 |
KTM writes gears as driving:driven teeth (13:32 is a 2.46 reduction). Honda prints decimal ratios only; the tooth counts in brackets are our inference from them. Honda also warns that its Torque Control is calibrated for the recommended tires and sprockets.
Picking A Change
- Small step. Change the rear by one or two teeth, about 2 percent each on a 50 tooth sprocket.
- Big step. Change the front by one tooth, about 7 to 8 percent on a 13 or 14.
- Same chain? Each rear tooth moves the axle roughly 4 mm on a 520 chain (our geometry calculation). KTM's adjusters cover 32 mm.
- Worn drivetrain? Replace the chain and both sprockets together, as KTM specifies. See the chain slack and wear guide.
FAQ
How do I calculate my final drive ratio?
Divide the rear sprocket teeth by the front sprocket teeth. 51 rear and 14 front is 51 / 14 = 3.64.
How much does one front tooth change gearing?
About 1 divided by the front tooth count. Going from 14 to 13 on a 51 rear is 7.7 percent shorter, about the same as adding 3.6 to 3.9 rear teeth.
How much does one rear tooth change gearing?
About 1 divided by the rear tooth count. Going from 51 to 52 with a 14 front is 2.0 percent shorter.
How is speed per gear calculated?
Speed equals engine rpm divided by the overall ratio (primary x gear x final drive), times the tire circumference. The calculator uses nominal tire sizes, so real speed is a little lower under load.
Does a bigger rear sprocket need a longer chain?
Each extra rear tooth needs about half a pitch more chain wrap, so the axle moves forward roughly 4 mm on a 520 chain. KTM's adjusters have 32 mm of range for different secondary ratios; beyond that you change the chain length.
What does shorter gearing mean?
A numerically higher final drive ratio, from a bigger rear or smaller front sprocket. It pulls harder and gives less speed per gear.
Log Every Sprocket Swap
MotoMind keeps a mod log per bike and shows chain inspection and chain and sprocket replacement as upcoming, due or overdue by engine hours. Available to download on iOS.
