RPM to Speed Calculator

This calculator converts engine speed into vehicle speed from three things: the ratio of the gear you are in, the final drive (differential or axle) ratio, and the overall tire diameter. It also shows wheel RPM and tire circumference so you can check each step.

It is used when choosing an axle ratio for a truck or project car, checking what RPM a highway cruise will run at after fitting larger tires, plotting top speed in each gear for track driving, or confirming that a gearbox swap suits the engine's power band.

The result is a theoretical speed with no clutch or tire slip. Cars with a torque converter that has not locked up will travel a little slower than shown, and loaded tires have a slightly smaller rolling radius than their unloaded diameter. If you only know your tire's size code, the tire size calculator gives its diameter.

Collecting the constants gives the common shortcut mph = RPM × D ÷ (gear × final × 336.13). Rearranged, it gives engine RPM at a chosen speed: RPM = mph × gear × final × 336 ÷ D. Multiply mph by 1.609 for km/h. For a transfer case or a two-speed axle, multiply its ratio into the gear term. The method is basic gear-train kinematics as covered in the Bosch Automotive Handbook.

A metric size such as 225/45R17 is width in mm / aspect ratio (sidewall height as a percentage of width) / rim diameter in inches. Diameter in inches = (2 × width × aspect ÷ 100) ÷ 25.4 + rim.

• 225/45R17: 2 × 225 × 0.45 = 202.5 mm of sidewall = 7.97 in, plus 17 = 24.97 in.
• 265/70R17: 2 × 265 × 0.70 = 371 mm = 14.61 in, plus 17 = 31.6 in.

Light-truck sizes written like 33x12.50R15 already give the diameter (33 in) first. Actual tires can measure slightly under their nominal size, and a worn tread loses up to about half an inch of diameter.

The calculator gives the speed the drivetrain geometry implies. Speedometers are allowed to over-read: UN ECE Regulation 39, used in the UK and EU, permits a speedometer to show more than the true speed but never less. Fitting taller tires than stock makes the car travel further per wheel turn, so the speedometer then reads low. The speedometer error calculator works out that difference directly, and the gear ratio calculator helps when comparing axle ratios.

A numerically higher final drive (4.10 versus 3.42) multiplies torque for towing and acceleration but raises cruising RPM and usually fuel use. A lower ratio does the opposite. The trade-off is easiest to see by running the same speed through both ratios: a direct-drive (1.0) top gear with a 3.42 axle and 24.97 in tires gives about 54 mph at 2,500 RPM. Many engines are most efficient on the highway somewhere around their peak-torque RPM or a little below, so aim the cruising RPM near that range rather than as low as possible.