Formula
Sidewall height is the section width times the aspect ratio. The diameter is the rim plus two sidewalls. Ring teeth divided by pinion teeth is the axle ratio. If the pinion count is zero, the axle-ratio field is used instead.
sidewall inches = width mm × (aspect ÷ 100) / 25.4
diameter = rim + 2 × sidewall
axle = ring ÷ pinion
rpm = mph × axle × gear × 63360 / (60 × π × diameter)
63,360 / (60π) is 336.135, the engine revolutions per mile of a 1-inch tire in a 1:1 overall ratio, per mph. A transmission gear of 1.00 is direct drive. The small table under the result is the same axle at a few example gear ratios, so you can see first through overdrive without retyping. Those ratios are examples, not your transmission.
Worked examples
265/70R17, 41/11, gear 1.00, 60 mph. Diameter 31.61 in. Axle 3.727. Engine speed 2,378.4 rpm.
The same truck in a 3.06 first gear at 30 mph. Overall ratio 3.727 × 3.06. RPM is higher than the 60 mph direct-drive figure even though the road speed is half, because 3.06 is more than twice 1.00.
A 0.70 overdrive at 70 mph. RPM is 0.70 times the direct-drive RPM at that speed. The page does that as soon as you type 0.70 in the gear field.
Reference
Nominal diameter for a few common sizes. RPM uses a 3.73 axle and a 1.00 gear at 60 mph.
| Tire | Diameter | RPM at 60 |
|---|---|---|
| 265/70R17 | 31.61 in | 2,380 |
| 245/75R16 | 30.47 in | 2,469 |
| 285/70R17 | 32.71 in | 2,300 |
| 225/45R17 | 24.97 in | 3,012 |
Questions
What RPM is a 265/70R17 at 60 mph with 3.73 gears in direct drive?
About 2,380 rpm. The sidewall is 265 × 0.70 / 25.4 = 7.303 inches, so the diameter is 17 + 14.606 = 31.606 inches. RPM = 60 × 3.73 × (63,360 / (60π)) / 31.606. 41/11 is 3.727, the same 3.73 to two decimals.
What does the transmission gear do?
It multiplies the axle ratio. First gear at 3.00 with a 3.73 axle is an overall ratio of 11.19, so the engine turns 3 times as fast as it does in a 1.00 gear at the same speed. Overdrive under 1.00 lowers the RPM.
Is the tire diameter exact?
It is the nominal diameter: rim plus two sidewalls from the size molded on the tire. A worn or loaded tire is shorter, and RPM climbs a little. This page does not measure revolutions per mile from a manufacturer table.
Sources
- Tire diameter from the molded size: rim + 2 × width × aspect / 25.4. Nominal, unloaded.
- RPM = mph × axle × transmission gear × 63360 / (60 π diameter). 63,360 inches in a mile.