Gear Ratio and RPM at Speed Calculator

Find engine RPM at highway speed from your axle ratio, transmission gear and tire size, or work backwards to speed or axle ratio.

What Do You Want to Find?
Use the tire size calculator for metric sizes.
Top gear: 1.00 for direct drive; overdrive is under 1.
0 if locked up or manual.
Fractions and decimals both work.
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RPM
Engine RPM
Speed
Axle ratio
Overall ratio
Wheel RPM
Per Mile

Gearing Setups

Items you add are saved in this browser.

SetupTireAxleGearRPM at 70Remove

How to Calculate RPM at Speed

Engine RPM follows the tires through the axle and transmission gears. At 1 mph, a tire turns 1 mile ÷ its circumference each hour; the gears multiply that up to the engine.

RPM = mph × axle ratio × transmission ratio × 336.13 ÷ tire diameter (in)
MPH = RPM × tire diameter ÷ (axle ratio × transmission ratio × 336.13)

336.13 is 63,360 inches per mile ÷ 60 minutes ÷ π. Torque converter slip raises RPM by the slip percentage.

Worked Example

RPM at 70 MPH on 31″ Tires in 0.67 Overdrive

Axle RatioRPM

Tips

After a tire size change, re-gearing the axle in proportion to the diameter change brings RPM back to where it was: new ratio = old ratio × new diameter ÷ old diameter. Measure a loaded tire's height for the most accurate numbers; tires squat a little under the vehicle's weight.