Vehicle · Drivetrain
Re-gear ratio for a new tire size calculator.
What axle ratio matches your old gearing with a taller (or shorter) tire.
Inputs
Gear ratio
Enter the details to see the result.
What gear ratio do I need for bigger tires?
The effective gearing at the road varies inversely with tire diameter. To keep the original effective ratio, multiply the axle ratio by the new diameter divided by the old diameter: new ratio = old ratio x (new diameter / old diameter). Fitting a 35 inch tall tire in place of a 31.6 inch one (265/70R17) on a 3.73 axle gives 3.73 x 35 / 31.6 = about 4.13, so a 4.10 set is the closest common choice. The percentage change in effective ratio if you keep the stock gears is 1 / (new / old) - 1, so the same tire change drops the effective ratio by about 9.7%, from 3.73 to 3.37.
Re-gearing restores engine speed and leverage, but it does not correct the speedometer error that comes from the tire size, and on some vehicles the speedometer signal also depends on the gears. After any change, compare the speedometer with GPS and recalibrate if the vehicle allows.
Frequently asked questions
How do I calculate the new gear ratio for larger tires?
Multiply the current axle ratio by the new tire diameter divided by the old. 3.73 x (35 / 31.6) is about 4.13, so a 4.10 gear is close.
What happens if I keep the stock gears?
The effective ratio falls by the same percent that the tire grew. A 10.7% taller tire cuts the effective ratio about 9.7%, so the engine turns slower at a given speed and has less leverage.
Does re-gearing fix the speedometer?
Not by itself. The speedometer error comes from the tire size, and how your vehicle senses speed decides what else matters, so compare against GPS and recalibrate if the vehicle supports it.
Why not always match the ratio exactly?
Gear sets come in fixed steps, such as 3.73, 4.10 and 4.56, so you choose the nearest ratio that suits your driving and your axle's available sets.
Sources
Checked
Scope
Gearing arithmetic only. Axle, ring-and-pinion and driveline choices need a qualified mechanic or the axle maker's data.