Speed from RPM and Gearing Calculator
Calculate vehicle speed from engine RPM, gear ratio, final drive and tire diameter — or find the RPM at a given cruise speed.
How it works
Wheel RPM = engine RPM ÷ (gear × final drive) · Speed (mph) = wheel RPM × π × tire diameter (in) × 60 ÷ 63,360
Engine speed reaches the wheels divided down by the overall ratio — the transmission gear times the final drive — so the wheels turn slower than the crank. Each wheel revolution rolls the tire’s circumference (π × diameter) forward, and multiplying by wheel RPM gives distance per minute, converted to miles per hour. Taller tires or taller gearing lower cruising RPM for the same speed, improving economy and refinement but blunting acceleration.
Worked example
At 3,000 rpm in a 1.00:1 gear with a 3.73 final drive, the wheels turn 3,000 ÷ 3.73 ≈ 804 rpm. On a 25-inch tire (78.5 in circumference) that is 804 × 78.5 ≈ 63,150 in/min, which is about 59.8 mph.
Frequently asked questions
How do I find my tire diameter?
Read the sidecar size like 225/45R17 and compute diameter = rim × 25.4 + 2 × (width × aspect ÷ 100) in mm, then divide by 25.4 for inches — or use a tire-size calculator. Alternatively, roll the car one wheel turn and measure the distance to get circumference.
Why does my engine rev higher than this at speed?
You may not be in top gear, your final drive may be numerically higher (shorter) than entered, or your tires may be smaller than assumed. Any of these raises RPM for a given speed. Enter the exact gear ratio you are cruising in.
Do bigger tires make the speedometer read wrong?
Yes. A taller tire travels farther per revolution, so the car actually goes faster than the speedometer shows unless it is recalibrated. This calculator uses true tire diameter, so it reflects real road speed.
Related calculators
Educational estimate — real-world figures vary with load, temperature, traction, tuning and measurement method.