How to use this calculator
- Enter target RPM. Set the engine RPM you want at cruise or trap speed.
- Enter road speed and tire diameter. Use measured tire diameter when possible.
- Enter gear ratios. Add transmission and transfer-case ratios.
- Read final drive. Use the required final drive ratio as the axle ratio target.
How it works
The calculator first finds wheel RPM from road speed and tire circumference, then works backward to the drivetrain ratio: final drive = engine rpm / wheel rpm / (gear x transfer).
Use the engine RPM calculator to check an existing axle ratio, or the torque converter slip calculator when measured RPM is higher than the locked mechanical value.
Worked example
Verified against the live calculator
A 26 in tire at 65 mph turns about 840 rpm. With a 0.75 overdrive and 2,500 rpm cruise target, the required final drive is about 3.97:1.
Frequently asked questions
How do you calculate final drive ratio?
Calculate wheel RPM from road speed and tire circumference, divide target engine RPM by wheel RPM to get overall ratio, then divide by transmission and transfer-case ratio.
Is final drive ratio the same as axle ratio?
For most rear-drive and axle-based setups, yes. Transaxles may use final drive terminology instead of axle ratio.
Should I use tire diameter or revs per mile?
Measured tire revs per mile is best for precision. Tire diameter is a useful first-pass input.
Does this include torque converter slip?
No. It assumes the drivetrain is locked with no clutch or converter slip.
Method & assumptions
- Assumes no clutch slip, torque-converter slip, tire growth or rolling-radius change.
- Measured tire revs per mile is better than nominal tire diameter for final calibration.