The formula
Engine RPM = speed × revolutions per mile ÷ 60 × axle ratio × transmission gear ratio
This is the exact form of the well-known shortcut RPM = mph × axle ratio × 336 ÷ tire diameter (in inches); 336 is 63,360 inches per mile divided by 60 minutes and by π.
Worked example
A truck on 265/70R17 (31.6 in) with 3.73 gears in a 0.69 overdrive at 65 mph turns 65 × 638.0 ÷ 60 × 3.73 × 0.69 ≈ 1,779 rpm. On 35x12.50R17 tires the engine turns about 1,600 rpm, as if the axle ratio were 3.37. To get the original RPM back you would need about 4.13 — in practice 4.10 gears.
Re-gearing chart from 265/70R17 (31.6 in)
| New tire | Was 3.42 | Was 3.73 | Was 4.10 |
|---|---|---|---|
| 33x12.50R17 | 3.57 (≈3.55) | 3.89 (≈3.92) | 4.28 (≈4.30) |
| 35x12.50R17 | 3.79 (≈3.73) | 4.13 (≈4.10) | 4.54 (≈4.56) |
| 37x12.50R17 | 4.00 (≈3.92) | 4.37 (≈4.30) | 4.80 (≈4.88) |
Values in brackets are the nearest commonly available ring-and-pinion ratio.
Frequently asked questions
Why does engine RPM drop with bigger tires?
A taller tire covers more ground per revolution, so at the same road speed the wheels — and the engine connected to them — turn more slowly. It feels like a taller (numerically lower) axle ratio.
Where do I find my axle ratio?
On many US trucks it is on the axle tag or in the build sheet, often printed on the door-jamb sticker as an axle code. Your owner's manual or dealer can decode it.
What gear ratio should I use for the transmission?
Use the gear you cruise in. Overdrive top gears are usually between about 0.60 and 0.85; direct drive is 1.00. If you are unsure, the ratios are in your transmission's specifications.
Do I have to re-gear after fitting bigger tires?
Not always. Going up about 1 inch rarely needs it. Bigger jumps can leave the engine below its best RPM, hurting acceleration, towing and sometimes fuel economy, so many owners re-gear to restore the original effective ratio.
Is the speedometer affected too?
Yes. The speedometer reads low by the same percentage the tire diameter increased, unless it is recalibrated — see the tire size comparison calculator.