What the inputs mean
- Tire: the size on the sidewall or the measured overall diameter. The calculator uses the nominal diameter; a loaded tire rolls slightly smaller, so real rpm is often a little higher.
- Axle ratio: the ring and pinion ratio in the differential, for example 3.73.
- Transmission gear ratio: the ratio of the gear you are in. Top gear on many automatics is an overdrive below 1.00. The owner's manual or the transmission maker lists each gear.
- Transfer case ratio: only for four-wheel drive in low range. High range is 1.00.
The results assume no slip. With a torque converter that is not locked, or a slipping clutch, the engine turns faster than shown.
The formula
One mile is 63,360 inches, and the tire covers its circumference (π × diameter) each turn. Per minute, that gives:
rpm = mph × 336.14 × axle ratio × gear ratio × transfer ratio ÷ tire diameter (in)336.14 is 63,360 ÷ (60 × π). Turn it around to get the speed at a given rpm:
mph = rpm × tire diameter ÷ (336.14 × axle ratio × gear ratio × transfer ratio)Worked example
A truck on 265/70R17 tires (31.61 in) with 3.73 axle gears, in a 1.00 direct gear, at 65 mph: 65 × 336.14 × 3.73 × 1.00 ÷ 31.61 = 2,578 rpm. In a 0.70 overdrive the same speed takes 1,805 rpm. At 3,000 rpm in the 1.00 gear the truck does 75.6 mph.
In low range the numbers change a lot. With a 4.00 first gear and a 2.50 transfer case low range, 5 mph takes 1,983 rpm on the same tires and axle.
Bigger tires
A taller tire lowers the rpm at every speed, the same as a lower axle ratio. If you changed tires, compare the two sizes here, then see which axle ratio restores the old rpm with the gear ratio calculator.
Questions
How do I calculate engine RPM from speed?
Multiply the speed in mph by 336.14, the axle ratio and the gear ratio, then divide by the tire diameter in inches. 65 mph on 31.61-inch tires with 3.73 gears in a 1.00 gear gives 2,578 rpm.
What RPM should I be at on the highway?
There is no single right number; it depends on the engine. This calculator shows what your gearing gives: on 31.61-inch tires with 3.73 gears and a 0.70 top gear, 70 mph is 1,944 rpm. Your owner's manual may say more about the engine.
How fast will I go at 2,000 RPM?
Use the speed at an rpm card. On 31.61-inch tires, 3.73 gears and a 0.70 overdrive, 2,000 rpm is 72.0 mph.
Do bigger tires lower RPM?
Yes. Engine rpm falls in proportion to the tire diameter: a tire 10% taller turns the engine about 9% slower at the same speed.