How it works
A taller tire covers more ground per revolution, so the whole driveline turns slower for the same road speed.
Effective ratio = axle ratio × original diameter ÷ new diameter
Change in wheel torque = original diameter ÷ new diameter − 1
Ratio to restore gearing = axle ratio × new diameter ÷ original diameter
Actual speed at an indicated 60 = 60 × new diameter ÷ original diameter
Worked example
A truck with 3.73 gears goes from 31 in to 33 in tires.
- Effective ratio: 3.73 × 31 ÷ 33 = 3.50
- Torque at the wheels: about 6% less
- To restore the original feel: 3.73 × 33 ÷ 31 = 3.97, so a 4.10 is the usual choice.
- Speedometer: shows 60 at about 63.9 mph.
Common tire upgrades with 3.73 gears
| Tires | Effective ratio | Torque change | Ratio to restore |
|---|---|---|---|
| 31 → 33 in | 3.50 | -6.1% | 3.97 |
| 31 → 35 in | 3.30 | -11.4% | 4.21 |
| 32 → 35 in | 3.41 | -8.6% | 4.08 |
| 33 → 37 in | 3.33 | -10.8% | 4.18 |
| 35 → 37 in | 3.53 | -5.4% | 3.94 |
The verdict is “small change” up to 3% difference in diameter, “noticeable” up to 7% and “large” beyond that.
Towing with bigger tires
Losing 6–12% of wheel torque is felt most when starting on a hill, climbing grades and towing heavy. The transmission may also hunt between gears more often. Taller tires usually weigh more and have taller sidewalls, which can add sway with a trailer. Check that the new tires’ load rating covers your axle loads, and consider regearing if you tow near your limits.
Find tire diameters from the size on the sidewall with the tire size calculator, and check cruising RPM with the new effective ratio in the engine RPM calculator.
Common mistakes
- Using nominal sizes only. Measured diameters can differ from the size on the sidewall.
- Assuming the tow rating stays meaningful. Ratings were set with the original gearing and tires.
- Forgetting the speedometer. It affects tickets, odometer readings and fuel economy calculations.
- Mixing inches and millimeters. Use the same unit for both tires.