Speedometer Gear Calculator
Calculate vehicle speed from RPM, gear ratio, axle ratio, and tire diameter.
Use it for speedometer calibration, custom tire fitments, and axle ratio swaps.
The formula connecting engine RPM to road speed is:
Speed (mph) = (RPM × tire diameter (inches) × π) / (gear ratio × axle ratio × 1056)
The constant 1056 comes from unit conversion: 12 in/ft × 5,280 ft/mile ÷ 60 min/hr = 1,056.
What is happening physically: the engine turns at RPM rev/min. After the transmission (at gear_ratio) and the differential (at axle_ratio), the wheels turn at RPM / (gear_ratio × axle_ratio) rev/min. Each wheel revolution covers π × tire_diameter inches. Convert to miles per hour and you get the formula above.
When this matters
Stock vehicles are calibrated at the factory for specific tire and gear combinations. Change either and the speedometer reads wrong.
A common example: swapping from 265/70R17 (31.6 inch diameter) to 285/75R17 (33.8 inch diameter) on a truck makes the speedometer read about 7% low. At an indicated 60 mph you are actually doing about 64 mph.
Changing axle ratios also shifts the calibration. Going from 3.55 to 4.10 gears changes the speedometer reading in 5th gear by about 13% if the sensor is driven off the output shaft.
Finding tire diameter
Tire diameter is not printed directly on the sidewall. For a tire marked 265/70R17: diameter = 17 inch rim + 2 × (265 × 0.70 / 25.4) = 17 + 14.57 = 31.57 inches. Most tire manufacturers publish overall diameter in their spec sheets.
Gear ratios
1.0 is direct drive (common in top gear on many manual transmissions). Overdrive gears like 0.70 or 0.83 are less than 1.0 and lower RPM at highway speed. First gear is typically 2.5-4.0:1. Enter the ratio for whichever gear you want to analyze.
Common axle ratios: 3.08, 3.23, 3.31, 3.55, 3.73, 4.10. Numerically higher ratios give more low-end torque multiplication at the cost of lower top speed.