Vehicle · Drivetrain
Engine RPM at speed calculator.
RPM from road speed, tire size, axle ratio and gear. Compare two tire sizes.
Inputs
Engine RPM
Enter the details to see the result.
How do you calculate engine RPM from speed and tire size?
RPM = road speed in inches per minute / tire circumference in inches x axle ratio x transmission gear ratio. With speed in mph and tire diameter in inches this becomes RPM = mph x 1056 x ratio / (pi x diameter), where 1056 is 63,360 inches per mile divided by 60 minutes. Example: 65 mph in a 31.6 inch tall tire (265/70R17) with a 3.73 axle and 1.00 gear is 65 x 1056 / (pi x 31.6) x 3.73 = about 2,578 rpm. Metric speed uses 1,000,000 mm per km.
A taller tire covers more ground per revolution, so RPM at the same speed falls by the ratio of the diameters. Changing from 265/70R17 to 285/75R17 lowers RPM by about 6.6%. The overall ratio is the axle ratio times the gearbox ratio of the gear you are in, so look up your transmission's gear ratio in the owner's manual or from the maker. This is a drivetrain calculation, and automatic transmissions with torque converters run a little above it when the converter slips.
Frequently asked questions
What is the formula for RPM from speed?
RPM = mph x 1056 x overall ratio / (pi x tire diameter in inches). Overall ratio is the axle ratio times the transmission gear ratio.
How much does a bigger tire lower RPM?
RPM at a given speed changes by the inverse ratio of the diameters. A 285/75R17 is about 7% taller than a 265/70R17, so RPM is about 6.6% lower.
What gear ratio do I use for top gear?
Use your transmission's ratio for that gear, which the vehicle maker publishes. Direct drive is 1.00 and overdrive is below 1, for example 0.75.
Is the answer exact?
It is an estimate. Torque converter slip, tire wear, load and pressure change the real rolling circumference and engine speed slightly.
Sources
Checked
Scope
Estimate from nominal tire size and the ratios you enter. Not a substitute for a tachometer or the maker's data.