How tire size affects gearing
Your axle ratio (for example 3.73:1) is how many times the driveshaft turns for each turn of the wheels. Bigger tires travel further per turn, so at the same road speed the wheels, driveshaft and engine all turn more slowly. That feels like switching to a "taller" (numerically lower) axle ratio: lower RPM on the highway, but less acceleration and pulling power, more downshifting on hills, and sometimes worse fuel economy.
The formulas
The constant 336 (more precisely 336.13) comes from converting miles per hour to inches per minute and dividing by π: 63,360 ÷ (60 × π).
Worked example
A truck with 3.73 gears and 265/70R17 tires (31.61 in) moves to 35x12.50R17 tires (35 in), a 10.7% increase.
In a 1.00 top gear at 65 mph: RPM = 65 × 3.73 × 1.00 × 336.13 ÷ 31.61 ≈ 2,578 before, and ≈ 2,328 after.
The effective axle ratio drops to 3.73 × 31.61 ÷ 35 ≈ 3.37. To restore the original RPM you would need 3.73 × 35 ÷ 31.61 ≈ 4.13. The closest common ratios are 4.10 and 4.11.
Should you regear?
Regearing means replacing the ring and pinion gears in each axle. It is a precise, fairly expensive job for a specialist driveline shop, and on four-wheel-drive vehicles both axles must be changed to the same ratio. People usually regear when:
- the effective ratio has dropped by roughly 10% or more, as in the example above;
- the transmission keeps hunting between gears or the vehicle struggles in top gear on the highway;
- they tow, carry heavy loads or drive off-road and want low-speed control back.
Many drivers choose a ratio slightly higher than the exact "restore" value to make up for the extra weight of larger tires and wheels. Which ratios you can actually buy depends on your axle model, so the common ratios listed here are a starting point for a conversation with a shop, not a guarantee of availability.
Finding your numbers
- Axle ratio: often on a tag on the differential cover, in the vehicle's build sheet, or decoded from an axle code on a door-jamb or glovebox sticker.
- Top gear ratio: listed in the owner's manual or transmission specifications. Many modern automatics have overdrive top gears between about 0.6 and 0.8.
- Tire diameter: use the manufacturer's measured diameter if you know it; otherwise the calculated size is a good estimate.
The RPM formula assumes a locked torque converter or no clutch slip, which is typical at steady highway speeds. Speedometer error also changes with tire size; see the speedometer calculator.
Frequently asked questions
What gear ratio do I need for 35 inch tires?
It depends on your original tire and gears. As an example, going from about 31.6 in tires with 3.73 gears to 35s needs about 4.13 to restore the original RPM, so 4.10 or 4.11 gears are the usual choice. Use the calculator with your own numbers.
Do bigger tires lower RPM?
Yes. At the same speed, a taller tire turns more slowly, so the engine RPM drops in proportion to the diameter increase.
What does 336 mean in the gear ratio formula?
It converts miles per hour and tire diameter into revolutions per minute: 63,360 inches per mile divided by 60 minutes and by π gives about 336.13.
Is a higher or lower gear ratio better for bigger tires?
Bigger tires usually call for a numerically higher ratio (for example from 3.73 to 4.10) to bring back acceleration and towing performance.
Do I have to regear both axles on a 4x4?
Yes. Front and rear axles must have the same ratio on a four-wheel-drive vehicle, or the drivetrain can bind and be damaged.
Last reviewed November 2026. Spotted an error? Tell us.