GEARING TOOL

RPM & Speed Calculator

Estimate road speed from engine RPM, tire diameter, transmission ratio and final-drive ratio.

Drivetrain

Engine RPM

Tire diameter (in)

Transmission gear ratio

Final-drive ratio

Target road speed (mph)

Results

Enter your numbers and calculate.

Gearing formula

MPH ≈ RPM × tire diameter ÷ (gear ratio × final drive × 336)

The constant 336 converts inches and minutes into miles per hour. The reverse calculation estimates engine RPM at a selected road speed.

Why real RPM can differ

Automatic transmission converter slip, clutch slip, tire growth and actual loaded tire diameter can move real-world results away from the theoretical number.