RPM Calculator

Engine speed is fixed by road speed, tire diameter and the two ratios between the crankshaft and the road. Enter the speed, the tire size as it is marked on the sidewall, the gear ratio and the final drive, and you get crankshaft rpm, wheel rpm, revolutions per mile and mph per 1000 rpm. Two further modes run the chain backwards, giving road speed at a chosen rpm, or the axle ratio that would put the engine at a chosen cruising rpm. Tire slip and torque converter slip are not modelled, so an unlocked automatic will sit a little above these figures.

Calculator Type
Speed Unit

Road speed, tire size and the two ratios give the crankshaft speed.

The sidewall marking (225/45R17), a flotation size (31x10.50R15), or the overall diameter in inches (28.5).
The differential or transaxle ratio, the one quoted as 3.73:1. Enter 3.73, not the whole 3.73:1.
The ratio of the gear you are in. Direct drive is 1.00, an overdrive top gear is below 1.00, first gear is often 3 to 4.
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The same chain read backwards: how fast the car is going at a given crankshaft speed.

The sidewall marking (225/45R17), a flotation size (31x10.50R15), or the overall diameter in inches (28.5).
The differential or transaxle ratio, the one quoted as 3.73:1. Enter 3.73, not the whole 3.73:1.
The ratio of the gear you are in. Direct drive is 1.00, an overdrive top gear is below 1.00, first gear is often 3 to 4.
Widget
Speed Unit

Choosing a differential ratio: the one that puts the engine at the rpm you want at the speed you cruise at.

The sidewall marking (225/45R17), a flotation size (31x10.50R15), or the overall diameter in inches (28.5).
The ratio of the gear you are in. Direct drive is 1.00, an overdrive top gear is below 1.00, first gear is often 3 to 4.
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