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TireMath HQ

Drivetrain

Gear Ratio Calculator

A taller tire is a taller final drive. Fit 35s on a truck geared for 31s and every ratio in the drivetrain is effectively 12.9% longer, which is why it feels lazy and why the transmission hunts. This works out the axle ratio that puts the engine back where it was, and shows the cruising rpm before and after.

: 1
mph
Common sizes

Axle ratio to restore stock rpm

3.93 : 1

Nearest production set is 3.90. Going from 265/70R17 (31.61″) to 35x12.50R17 (35.00″) is a +10.74% change in diameter, and the axle ratio has to move by the same proportion to keep the engine where it was.

The arithmetic

  1. Required = stock ratio × (new diameter ÷ old diameter)
  2. = 3.55 × (35.00 ÷ 31.61)
  3. = 3.55 × 1.1074 = 3.931 : 1

Engine rpm at 70 mph, 1:1 top gear

Engine speed before and after the tire change, and after regearing
SetupRPMvs stock
Stock: 265/70R17 on 3.552,643—
New tires, stock gears2,387−256
New tires on 3.902,622−21

Production ratios available: 2.73, 3.08, 3.21, 3.31, 3.42, 3.55, 3.73, 3.90, 4.10, 4.30, 4.56, 4.88, 5.13, 5.38, 5.71, 6.17.

Ring-and-pinion sets come in fixed ratios, so the nearest available gear is what you can actually buy. The exact figure is shown so you can see how far off the nearest set lands.

Why do bigger tires change the gearing?

The axle ratio tells you how many times the driveshaft turns for one turn of the wheel. What matters at the road is that ratio combined with how far the wheel travels per turn — and that is rolling circumference, which is set by tire diameter.

So diameter and axle ratio are interchangeable levers on the same quantity. A 10.7% taller tire has the same effect as making the axle ratio 10.7% numerically lower. Restoring the original relationship means multiplying the stock ratio by exactly the diameter ratio — nothing more complicated than that.

The rpm formula, and where 336 comes from

Engine rpm in a direct 1:1 gear is mph × 336 × axle ratio ÷ tire diameter. The constant is a unit conversion, derived rather than measured: a mile is 63,360 inches, an hour is 60 minutes, so one mph is 1,056 inches per minute. Divide by π to turn inches of travel into tire revolutions and you get 336.135.

At 70 mph on 31.6-inch tires with 3.73 gears, that is 70 × 336.135 × 3.73 ÷ 31.6 = 2,777 rpm. Put 35-inch tires on without regearing and the same 70 mph drops the engine to 2,508 rpm — quieter, but well below where a truck engine makes useful torque under load.

Common questions about regearing

How do you calculate the gear ratio needed for bigger tires?
Multiply your stock axle ratio by the new tire diameter divided by the old one. With 3.55 gears on 31.6-inch tires, moving to 35-inch tires needs 3.55 × 35 ÷ 31.6 = 3.93, so the nearest production set is 3.90.
What rpm will my engine turn at a given speed?
In a 1:1 top gear, rpm equals mph × 336 × axle ratio ÷ tire diameter in inches. The 336 is 63,360 inches per mile divided by 60 minutes and by π — it is not a fudge factor, it is the unit conversion.
Do bigger tires make my truck feel slower?
Yes, and the mechanism is simple: a taller tire is a taller final gear. Going from 31.6 to 35 inches multiplies your effective ratio by 0.903, the same as swapping 3.55 gears for 3.21 — roughly a 10% loss of mechanical advantage at the wheel.
Does regearing fix the speedometer too?
Not by itself. Regearing restores engine rpm and drivability, but on most modern vehicles the speedometer is calibrated from wheel-speed sensors and the stored revolutions-per-mile value, which still needs correcting separately.

The diameter figures this uses come from the same calculation as the tire diameter calculator, and the speedometer side of the same change is covered by the speedometer error calculator.

Other tire maths on this site