RPM to Speed Calculator

Road speed depends on engine RPM, total drivetrain gear ratio (transmission × differential), and tire rolling diameter. Higher RPM or lower gearing increases speed for a given tire; larger tires raise speed at the same RPM.

This uses: speed (mph) ≈ (RPM × π × tire diameter in inches) ÷ (gear ratio × 1056). Enter total ratio — e.g., 3.73 diff only, or multiply trans gear × diff for in-gear cruising RPM checks.

Example: 3000 RPM, 3.42:1 ratio, 26 in tire → about 68 mph. Shift points and fuel economy planning often use this math — verify with a GPS for real-world calibration.

The 1056 constant in the formula is just unit bookkeeping: it converts inches per minute into miles per hour (63,360 inches per mile ÷ 60 minutes). Knowing that makes the relationship easy to sanity-check — double the tire diameter and you double road speed at the same RPM, double the gear ratio and you halve it.

Enter the total ratio, and be careful which number your spec sheet gives. A 3.42 axle ratio alone describes only top gear if that gear is 1.00:1. For any other gear, multiply transmission ratio by axle ratio: a 2.45:1 second gear behind a 3.42 axle is a total of 8.38:1, which is why second gear runs out so much sooner than the axle ratio alone suggests.

“Lower” and “higher” gearing are counterintuitive terms. A numerically higher axle ratio such as 4.10 is called lower gearing: it multiplies torque and improves acceleration while raising cruising RPM and hurting highway economy. A 3.08 axle is higher gearing, quieter and thriftier at speed but lazier off the line.

This is the calculation to run before changing tire size on a truck. Fitting a 33-inch tire in place of a 31-inch one raises road speed about 6% at a given RPM, which is equivalent to swapping to a numerically lower axle ratio — often enough to make the transmission hunt between gears on grades unless the axle is regeared to compensate.

Real speed lands slightly below the calculation because a driven tire deforms and slips a little under load, and because an automatic torque converter allows some slip unless it is locked up. Treat the result as the geometric ceiling, verify against GPS at steady cruise, and model each gear separately when comparing highway noise or shift points.

Final-drive ratio in the differential multiplies with transmission gear — confirm whether your spec sheet lists axle ratio or includes tire revolutions per mile already baked into a table value.

Overdrive top gear lowers RPM at cruise — model each gear separately when comparing highway noise and fuel economy targets.