Engine rpm from speed, axle ratio, transmission gear and tire size
Updated October 2026
The axle ratio (ring-and-pinion, or rear-end ratio) is on the axle tag, in the vehicle’s build sheet or option codes, or in the owner’s manual towing section; common values run from about 3.08 to 4.56. The transmission ratio is the gear you are in, from your transmission’s ratio chart: 1.00 in direct drive, below 1.00 in overdrive. Tire diameter should be the loaded height, measured from the ground to the center of the hub and doubled; it is usually a little less than the size printed on the sidewall. The results show engine rpm at your speed, the overall ratio, top speed at the rpm you enter, tire revolutions per mile, and what a new tire diameter does to rpm and effective gearing.
A tire of diameter D inches travels π × D inches per revolution, and a mile is 63,360 inches, so it turns 63,360 ÷ (π × D) times per mile. At a given speed in mph that is mph × 63,360 ÷ (60 × π × D) revolutions per minute at the wheels; the axle and transmission ratios multiply it up to engine speed. The constant 63,360 ÷ (60π) is 336.1.
At 70 mph with 3.73 gears, a 0.67 overdrive and 31-inch tires: 70 × 3.73 × 0.67 × 336.1 ÷ 31 = 1,897 rpm. Turned around, 2,200 rpm in that gear is 2,200 × 31 ÷ (3.73 × 0.67 × 336.1) = 81.2 mph.
| Axle ratio | RPM |
|---|---|
| 3.08 | 1,520 |
| 3.42 | 1,687 |
| 3.55 | 1,751 |
| 3.73 | 1,840 |
| 4.10 | 2,023 |
| 4.56 | 2,250 |
Taller tires act like a numerically lower axle ratio: the engine turns slower at the same speed, which can feel sluggish and leave an automatic transmission hunting for gears. Starting from 3.73 gears and 31-inch tires at 70 mph in a 0.67 overdrive:
| New tire | Effective axle ratio | RPM at 70 mph | Ratio to restore the original rpm |
|---|---|---|---|
| 31 in | 3.73 | 1,897 rpm | 3.73 |
| 32 in | 3.61 | 1,838 rpm | 3.85 |
| 33 in | 3.50 | 1,782 rpm | 3.97 |
| 35 in | 3.30 | 1,680 rpm | 4.21 |
| 37 in | 3.13 | 1,589 rpm | 4.45 |
To get back the original feel, multiply the old axle ratio by new diameter ÷ old diameter and choose the closest ratio offered for your axle.
Leaving out the transmission ratio. Using 1.00 when the truck is in a 0.67 overdrive overstates rpm by about half. Using the sidewall size instead of loaded height understates rpm slightly. Regearing only one axle on a four-wheel-drive vehicle. Front and rear ratios must match. Forgetting the speedometer. After a tire change, the speedometer reads low until it is recalibrated, so the "70 mph" you see is not 70. Reading rpm as the whole story. Engines make peak torque in a specific band; the right ratio keeps cruising rpm near it without over-revving.