Engine RPM at Speed Calculator
Find engine RPM at a given speed from tire diameter, axle ratio and transmission gear ratio.
How it works
Engine RPM = (mph × gear ratio × axle ratio × 336) ÷ tire diameter (in)
Engine speed at a given road speed is set by how far the tire travels per turn and how many engine turns the drivetrain takes for each wheel turn. The tire diameter fixes the wheel RPM at a given speed, and the transmission gear multiplied by the axle (final drive) ratio scales that up to the engine. The constant 336 bundles the unit conversions from miles per hour and inches into revolutions per minute (it is 5,280 ft × 12 in ÷ 60 min ÷ π, rounded). A 1:1 top gear makes the axle ratio the whole story; an overdrive gear below 1.0 lowers the cruising RPM further.
Worked example
At 65 mph on a 28-inch tire with a 3.73 axle ratio in a 1.0 top gear: RPM = (65 × 1.0 × 3.73 × 336) ÷ 28 ≈ 2,909. Slip into a 0.70 overdrive and the same speed drops the engine to about 2,036 RPM.
Frequently asked questions
Why does my engine rev lower in overdrive?
Overdrive gears have a ratio below 1.0, so the transmission output shaft spins faster than the engine. That reduces the engine RPM needed for a given road speed, which improves highway fuel economy and lowers noise.
How do bigger tires change cruising RPM?
A taller tire covers more ground per revolution, so the wheels — and therefore the engine — turn fewer times at a given speed. Fitting larger tires lowers your cruising RPM but also makes the speedometer read low unless it is recalibrated.
Where do I find my gear and axle ratios?
The axle (final drive) ratio is often on a tag on the differential or in the build sheet; transmission gear ratios are listed in the owner’s manual or service data. Use the top-gear ratio to find your highway cruising RPM.
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Educational estimate. Real-world results vary with conditions, tuning and measurement.