How to use this calculator
- Enter engine RPM. Set the engine speed you want to check.
- Enter tire diameter. Use nominal tire diameter or measured rolling diameter.
- Enter drivetrain ratios. Enter transmission, final drive and transfer-case ratios.
- Read road speed. Review road speed, wheel RPM and overall ratio.
How it works
Vehicle speed is the inverse of cruise RPM. The calculator divides engine RPM by the total drivetrain ratio, then multiplies by tire circumference: speed = engine rpm / (gear x final x transfer) x pi x tire diameter / 60.
Use the engine RPM calculator for the reverse calculation, or the tire size calculator if the tire diameter is changing.
Worked example
Verified against the live calculator
At 2,500 rpm with a 26 in tire, 0.75 overdrive and 3.73 final drive, the wheel is turning about 894 rpm and road speed is about 69 mph.
Frequently asked questions
How do you calculate vehicle speed from RPM?
Divide engine RPM by the overall drivetrain ratio to get wheel RPM, then multiply by tire circumference and convert from distance per minute to road speed.
What is overall drivetrain ratio?
Overall ratio is transmission gear ratio times final drive ratio times transfer-case or auxiliary ratio.
Does this include torque converter slip?
No. It assumes a direct mechanical connection. Add converter or clutch slip separately when it matters.
Should I use tire diameter or measured revs per mile?
Measured rolling diameter or manufacturer revs-per-mile data is better for precise speedometer work.
Method & assumptions
- Assumes no clutch slip, torque-converter slip, tire growth or rolling-radius change.
- Use measured rolling diameter or tire revs-per-mile for calibration-level work.