What the inputs mean
- Old tire: the tire the truck was geared for, usually the factory size. Type the size from the sidewall or the measured diameter in inches.
- New tire: the size you are fitting. The size code gives the nominal diameter; real tires can measure a little under it.
- Current axle ratio: the ring and pinion ratio, for example 3.55 or 3.73. It is often on a tag on the differential cover or in the build sheet. If you know the tooth counts, the card below turns them into a ratio.
The regear formula
A taller tire covers more ground per turn, so the engine turns slower at the same road speed. To get back the same engine speed, raise the axle ratio by the same proportion as the tire diameter:
new ratio = old ratio × new diameter ÷ old diameterThe reverse tells you how the truck feels before you regear, the effective ratio:
effective ratio = axle ratio × old diameter ÷ new diameterWorked example
A truck on 265/70R17 tires (31.61 in) with 3.55 gears moves to 35x12.50R17 (35.00 in). The target ratio is 3.55 × 35.00 ÷ 31.61 = 3.93. The closest common ratio is 3.90, which leaves the engine turning 0.8% slower than stock. 4.10 would make it turn 4.3% faster. Until you regear, the 3.55 gears act like 3.21 gears did on the old tires, 9.7% taller.
With a 0.70 top gear at 65 mph, the engine turns about 1,718 rpm on the old tires, 1,551 rpm on the 35s with 3.55 gears, and 1,704 rpm after a swap to 3.90. The RPM calculator shows every gear and speed, and the tire size comparison shows the speedometer error.
Picking between two ratios
If the target falls between two ratios, the lower number keeps the engine a little slower at cruise; the higher number gives more pull for towing, heavy loads and off-road work. This page does the maths only: it does not know your axle, transmission or how the truck is used, so check with the gear maker and your owner's manual.
Questions
What gear ratio do I need for 35 inch tires?
It depends on your old tire and ratio. From 31-inch tires on 3.73 gears, the maths gives 3.73 × 35 ÷ 31 = 4.21, so 4.10 or 4.30 are the closest common ratios. From 265/70R17 on 3.55, it gives 3.93, closest 3.90.
How do I work out my current gear ratio?
Divide the ring gear teeth by the pinion teeth. A 41-tooth ring and an 11-tooth pinion give 41 ÷ 11 = 3.727, sold as 3.73.
What gear ratio do I need for 33 inch tires?
Going from 31-inch tires on 3.73 gears to 33-inch tires, the maths gives 3.73 × 33 ÷ 31 = 3.97. The closest common ratio is 3.90, with 4.10 the next step up.
What happens if I do not regear after bigger tires?
The gearing gets taller by the same percentage as the tire. Going from 31.61 to 35 inches, 3.55 gears act like 3.21: the engine turns about 9.7% slower at the same speed, and the speedometer reads low until it is recalibrated.