Nasty Fast Parts • Tech Tool
Gearing v84
Compare sprocket combinations, tire size, transmission ratios and engine RPM. See theoretical speed in every gear, RPM at a chosen speed, and exactly how much taller or shorter a gearing change will be.
Saved BuildsShared with all NFP calculators
Saved builds remain on this browser. Export JSON to transfer between calculators or devices.
Machine
Start from a known transmission or enter your own ratios.
Transmission ratios
Sprockets
Compare what is on the machine now against a proposed setup.
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Tire & RPM
Use actual mounted tire diameter or measured rollout for better accuracy.
Nominal tire size is only approximate.
If entered, this overrides diameter.
Gearing Comparison
Calculated instantly from the values on the left.
Current gearing—Theoretical at selected RPM
New gearing—Theoretical at selected RPM
Real-world (estimated)—Power, drag & rolling resistance
Theoretical speed in each gear at selected engine RPM · new gearing
Current final drive—rear ÷ front
New final drive—rear ÷ front
Gearing change——
| Gear | Gear ratio | Current overall | Current speed | New speed | RPM @ target speed |
|---|
Graph uses the new sprocket setup. Theoretical speed does not account for tire growth, wheelspin, clutch slip, aerodynamic drag, or whether the engine has enough power to pull the calculated speed.
HP & BMEP Calculator
Read engine size, compression, porting and pipe RPM from the selected shared build. Estimates are editable.
Displacement—
Estimated BMEP—
Flywheel HP—
Wheel HP—
Select a shared build to estimate performance from Jetting or Engine Simulator inputs.
Power & Real-World Performance Estimate
Estimates wheel power vs air drag and rolling resistance on firm ground. Not a dyno result or a guarantee.
Uncheck to enter measured wheel-dyno power instead.
Derived from estimated flywheel HP with assumed drivetrain loss. Uncheck auto mode to enter measured wheel-dyno HP.
0.85 is a rough upright rider/ATV estimate.
Firm dirt approximation; deep sand will be much higher.
Loaded from the selected shared Jetting / Engine Simulator build where available.
Power falls off beyond peak RPM.
Load a shared build to model pipe RPM and engine parameters. Enter dyno wheel HP for better speed estimates.
Model: steady, level-ground speed, 1.225 kg/m³ air density, simplified RPM-dependent power curve, no wind or wheelspin. Power model is illustrative, and may overestimate attainable speed in low gears. For accurate results, enter wheel dyno power and test against GPS.
Quick Read
What the sprocket change does without making you think about the ratio backwards.
Speed change—same RPM
RPM change—same road speed
Current RPM—at target speed in 6th
New RPM—at target speed in 6th
Preset drivetrain data is provided as a convenience. Always verify unusual model-year combinations or aftermarket internal gearing before relying on calculated speed.