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Tire Size Math Tire size and gear ratio math.

Effective gear ratio and RPM calculator

See how a tire size change alters the effective axle ratio and engine RPM, and which ratio would bring the RPM back to where it was.

For example 3.73

1.00 for a 1:1 gear; overdrive is below 1 (for example 0.70)

Example: 3.73:1 with 265/70R17 to 285/70R17

Effective ratio
3.60:1
RPM at 65 mph
2,577 → 2,491
−3.4%
Ratio for the old RPM
3.86:1
nearest common 3.92:1

Transmission gear 1.00:1 (direct drive).

Engine RPM by speed

Road speed265/70R17285/70R17
30 mph1,1901,149
45 mph1,7841,724
60 mph2,3792,299
75 mph2,9742,874

The 336 constant assumes the transmission is in a 1:1 gear. Most modern transmissions have an overdrive top gear below 1:1 (often 0.6 to 0.85); enter it as the transmission ratio to include it.

Check the vehicle's door placard and owner's manual, and a tire professional, before changing sizes.

How it's calculated

A taller tire acts like a numerically lower gear: 3.73 gears on 285/70R17 behave like 3.60 gears on 265/70R17. Diameters are nominal design sizes from the size code.

Frequently asked questions

What gear ratio do I need for bigger tires?

To get the old cruising RPM back, multiply the current ratio by the new diameter and divide by the old one. For 3.73 gears going from 265/70R17 to 285/70R17 that is 3.86, so 3.92 is the nearest common ratio.

Why does the overdrive gear matter?

The 336 formula gives RPM in a 1:1 gear. In a 0.70 overdrive the engine turns 30% slower, so 2,577 RPM at 65 mph becomes about 1,804 RPM. Enter your top gear ratio to see the real cruising RPM.

Do bigger tires really change the gearing?

Yes. Each wheel turn covers more ground, so the engine turns fewer times per mile. That lowers cruising RPM and also reduces the torque reaching the ground, which is why acceleration and towing feel weaker.

Where do I find my transmission gear ratios?

The owner's manual, the transmission maker's spec sheet or a build sheet lists each gear. Common overdrive top gears are between about 0.60 and 0.85.

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