RimRatio

Gear ratio & tire size RPM calculator

Bigger tires change your effective gearing. See the engine RPM at any speed, the ratio your axle now "feels like", and the ratio that would restore the original overall gearing.

1.00 = direct drive

At 65 mph in a 1.00 gear with a 3.73 axle: 2,578 rpm on 265/70R17 vs 2,372 rpm on 315/70R17 (−206 rpm).

Effective axle ratio on 315/70R173.43 (feels like it)
Axle ratio to restore original gearing4.06
Nearest ratios on our reference list3.91 · 4.10

Arithmetic only (no torque-converter slip). Not a recommendation to change gears; available ratios depend on the axle.

Formulas

RPM = mph × axle ratio × gear ratio × 336.14 ÷ tire diameter (in)

effective ratio = axle ratio × old diameter ÷ new diameter

regear ratio = axle ratio × new diameter ÷ old diameter

Example: 65 mph, 3.73 axle, direct drive — 265/70R17: 2,578 rpm; 315/70R17: 2,372 rpm. Torque-converter slip is ignored. These are arithmetic results, not a recommendation to change gears.

Frequently asked questions

What do bigger tires do to gearing?

A tire 8.7% taller has the same effect on engine speed as an axle ratio 8.0% numerically lower (3.73 behaves like 3.43): the engine turns slower at a given road speed and the wheels receive less torque multiplication. Whether to change gears depends on the vehicle, its use and its maker's guidance — the calculator only shows the arithmetic.

What ratio restores my original performance?

Multiply your current axle ratio by new diameter ÷ old diameter. From 265/70R17 to 315/70R17 on 3.73 gears: 3.73 × 34.362 ÷ 31.606 = 4.06, the nearest ratios on our reference list are 3.91 and 4.10.

What is the 336 in the RPM formula?

It converts miles per hour and tire inches into revolutions per minute: 63,360 inches per mile ÷ (60 minutes × π) = 336.14.

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