Gear Ratio Speed Calculator
This gear ratio speed calculator converts engine RPM to vehicle speed using the transmission gear ratio, final drive (axle) ratio, and tire diameter. It is the standard formula used by automotive engineers and performance tuners to determine vehicle speed at any RPM in any gear. Whether you are evaluating a gearset swap, checking speedometer accuracy after a tire change, or plotting a shift strategy for a race, this tool gives you the exact speed in miles per hour. Enter your engine RPM, the gear ratio for the gear you are in, the rear axle ratio, and the overall tire diameter in inches. The calculator applies the SAE-standard relationship between rotational speed and linear velocity to produce vehicle speed in mph.
Gear ratio speed formula
Speed (mph) = (RPM * Tire Diameter) / (Gear Ratio * Axle Ratio * 336)
The constant 336 comes from the unit conversion: (60 min/hr) / (pi * 1/12 ft/in * 1/5280 mi/ft) = approximately 336. Tire diameter is in inches. Multiply by 1.60934 to convert mph to km/h.
How to use the gear ratio speed calculator
- Find your gear ratios in your owner's manual, the transmission manufacturer's specifications, or the build sticker in the door jamb.
- Find your axle ratio on the axle tag, build sheet, or owner's manual. Common passenger car ratios run from 2.73 to 4.10.
- Measure tire diameter with a tape measure from the ground to the top of the tire and double the result, or use a tire size calculator to convert a metric tire code (e.g., 245/55R17) to overall diameter in inches.
- Enter the RPM you want to evaluate. Repeat for different gears by changing the gear ratio input.
Frequently asked questions
What is a gear ratio?
A gear ratio is the ratio of the input gear teeth to the output gear teeth. A 3.5:1 first gear ratio means the engine turns 3.5 times for every one rotation of the transmission output shaft, providing more torque but lower speed.
How does tire diameter affect speed calculation?
A larger tire diameter increases the distance traveled per wheel revolution, so the vehicle moves faster at the same RPM. Changing tire size from the stock specification causes speedometer error that this calculator quantifies.
What is a final drive ratio?
The final drive ratio (also called axle ratio) is the ratio of the ring and pinion gears in the differential. It multiplies the transmission gear ratio. A 3.73 axle ratio means the driveshaft turns 3.73 times per wheel revolution.
Why does my calculated speed differ from my speedometer?
Speedometers are calibrated for a specific tire size. If you have installed non-stock tires, the actual vehicle speed and the speedometer reading will diverge. The formula uses the actual tire diameter to compute true speed.
What RPM should I shift at to maximize performance?
Shift points depend on your engine's power curve. Generally, shift near peak horsepower RPM (not peak torque) to maintain maximum acceleration. For street driving, shifting 500-1,000 RPM below redline balances performance and engine longevity.
Official sources
- SAE International: sae.org - vehicle dynamics and powertrain standards.
- NHTSA: nhtsa.gov - tire size and vehicle specifications.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.