Tire RulerHow regearing works
To keep the same effective gearing, the axle ratio has to grow by the same percentage as the tire diameter:
new ratio = old ratio × new tire diameter ÷ old tire diameter
Example: 265/70R17 → 35x12.50R17 with 3.73 gears
- Diameters: 31.6 in → 35.0 in.
- 3.73 × 35.0 ÷ 31.6 = 4.13, so choose 4.10 or 4.30.
- At 65 mph with a 0.69 top gear: 1,779 RPM before, 1,607 RPM on the new tires with stock gears.
Common axle ratios
Ring-and-pinion sets come in fixed steps. The usual ones are 3.08, 3.21, 3.31, 3.42, 3.55, 3.73, 3.92, 4.10, 4.30, 4.56, 4.88, 5.13, 5.29, 5.38, 5.71. Availability depends on your axle model, so check with a gear shop.
Frequently asked questions
Do I need to regear for 35-inch tires?
Not always, but going from about 31 in to 35 in makes your gears act about 10% taller. Trucks that tow, off-road or have small engines usually feel much better after regearing to restore the original effective ratio.
Which ratio should I choose between two options?
Pick the numerically higher ratio (e.g. 4.30 over 4.10) for towing, off-road and bigger future tires; pick the lower one for highway fuel economy and lower RPM.
What is the RPM formula?
RPM = mph × axle ratio × transmission gear ratio × 336 ÷ tire diameter in inches. 336 comes from 63,360 inches per mile ÷ (60 minutes × π).
Where do I find my axle ratio?
Look for the axle code on the door-jamb sticker, the tag bolted to the differential cover, or the original window sticker. A dealer can also look it up from your VIN.