Gear Ratio Calculator for Bigger Tires
Bigger tires change your effective gearing. Enter your original and new tire sizes and your current axle ratio to see how your gears really perform now — and the new ratio that restores your original power, acceleration, and towing. You can type a tire size (265/70R17, 35X12.50R17) or just a diameter in inches (33).
How re-gearing works
Your axle (differential) ratio sets how many times the driveshaft turns for each wheel revolution. The factory picks it to match the stock tire size. When you fit a larger tire, the wheel travels farther per turn, so the engine ends up spinning slower at any given speed — exactly as if someone installed taller, numerically-lower gears. The fix is to re-gear the axles to a numerically-higher ratio that cancels out the larger tire.
Reading the result
The effective ratio shows how your current gears behave with the new tires — this is the "you've lost this much" number. The recommended ratio is what restores your original feel, with the closest common off-the-shelf ratio beside it. Many builders round up a step for extra low-end torque with heavy tires or lockers; highway drivers often stay right at the number. Whatever you choose, both axles get the same ratio.
Diameter is the input that matters
Re-gearing math runs on overall diameter, not the printed size. A tire marketed as a "35" can measure anywhere from about 34.3 to 35 inches depending on construction, and that difference shifts the ideal ratio. Use a real measured or calculated diameter where you can — our tire sizes by diameter chart lists the true height of common sizes.
Frequently asked questions
Why do bigger tires hurt my gearing?
A larger tire has to turn fewer times to cover the same distance, so at any given speed your engine spins slower — effectively the same as installing taller (numerically lower) gears. You lose low-end torque, acceleration, towing grunt, and fuel economy, and the transmission hunts for gears. Re-gearing the axles to a numerically higher ratio restores the original relationship.
What gear ratio do I need for 35-inch tires?
It depends on your original tire size and factory ratio — there is no single answer. Enter your numbers above: the calculator multiplies your current ratio by (new diameter ÷ old diameter) to find the ratio that restores your original performance, then shows the closest common ratio. As a rough guide, going from ~32-inch stock tires to 35s on a truck often means moving from 3.73 or 3.55 up toward 4.10–4.56.
Should I round the recommended ratio up or down?
The calculator gives the ratio that restores your exact original feel. Many off-roaders round up to the next common ratio for a little extra low-end grunt (helpful with heavy tires, lockers, or frequent towing), while highway-focused drivers may stay at or just below it to keep rpm and fuel economy down. Both axles must be re-geared to the same ratio.
What is "effective gear ratio"?
Your effective ratio is how your current axle gears actually behave once the tire size changes: current ratio × (old diameter ÷ new diameter). Bigger tires lower it (feels taller/slower); smaller tires raise it. It is the single number that tells you how much the tire change moved your gearing.