How to use this calculator
Replace the example values with your own measurements or specifications. Keep the units shown beside each label. Results update as you type; reset restores the example.
Assumes no clutch or torque-converter slip and uses nominal tire diameter. Real rolling circumference, converter operation, or a CVT can make dashboard RPM differ.
The formula
A worked example
60 mph and a 26-inch tire are equivalent to 96.56064 km/h and a 660.4 mm tire. With a 0.75 transmission ratio and 3.73 final drive, either set of inputs gives approximately 2,170 rpm. You may also mix km/h with inches or mph with millimeters.
Common questions
Can I calculate RPM using km/h?
Yes. Select km/h for road speed and choose inches or millimeters independently for the tire diameter. Enter the number in the selected unit; changing the selector does not convert an existing value.
Is tire diameter the same as wheel size?
No. A 17-inch wheel sits inside a larger tire. Use the full outside tire diameter. Our Tire Size Calculator estimates nominal diameter from the tire code; manufacturer rolling-circumference data may differ.
What does an overdrive ratio below 1 mean?
The transmission output turns faster than its input, reducing engine speed for a given final drive and road speed.
Does this predict top speed?
No. It relates speed and RPM geometrically; available power, drag, limits, and traction are not modeled.
Sources & calculation method
KHK Gears: Internal Gears technical information (PDF) relates tooth-count ratio to input/output rotational speed. This calculator applies the speed-ratio relationship successively to the transmission and final drive.
For this model, each ratio means input rpm divided by output rpm. Road speed in mph × 5,280 × 12 ÷ 60 gives inches traveled per minute, which explains the 1,056 constant. Dividing by π × tire diameter gives wheel rpm. Multiplying by both ratios gives engine rpm. The familiar rounded shortcut uses 336 instead of 1,056 ÷ π ≈ 336.135; this tool keeps π.
Metric and mixed-unit inputs use the NIST length conversion factors: one inch is 25.4 mm and one mile is 1.609344 km. Convert speed to meters per minute and tire diameter to meters before dividing by π × diameter.
A ratio such as 3.73 means 3.73 driveshaft turns per wheel turn in the simplified straight-line model. Use the actual selected transmission ratio, not the number of gears. Tire deformation and drivetrain slip are outside this calculation.
References accessed September 9, 2026. These sources support the stated relationships and units; they have not reviewed or endorsed AutomotiveCalc.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.