Differential Gear Ratio to Tire Size Guide

by AJ Deysel

Comparing tire size to axle gear ratio and how that affects power and fuel economy.

About this Engine RPM / Tire Size chart:

This chart of Tire sizes and axle gear ratios provide a general idea of the relationship between the tire size and the differential gear ratio in an axle, and the effect that ratio has on fuel economy and power output. The chart is based on RPM(Engine Revolutions per Minute) levels at 65 mph (105 kph) and a drive train running at a 1:1 gearing ratio. This means the transmission and transfer case are passing power straight through at a one to one ratio without reducing or overdriving the engines rotations per minute before it reaches the differential. It is also worth noting that a manual transmission will be the most efficient at transferring power through to the axles and that an automatic transmission will have some inherent slippage without a locking torque converter. It is also worth noting that in an overdrive transmission, your engine RPM will be roughly 20 to 30% lower than shown here depending on that specific overdrive ratio.

Tire Diameter from Imperial Tire Size:

(CROSS SECTION WIDTH x 0.ASPECT RATIO x 2 ÷ 25.4) + WHEEL DIAMETER = OVERALL TIRE DIAMETER

For Example:

285/75R16

(285 x 0.75 x 2 ÷ 25.4) + 16 = 32.83″

Axle Gear Ratio Calculation:

RING GEAR TOOTH COUNT ÷ PINION GEAR TOOTH COUNT = GEAR RATIO

For example:

41-tooth ring gear ÷ 10-tooth pinion gear = 4.10

Whether you are building your daily truck, trail Jeep, rock crawler or drag racer, you now have the info to get your axle gearing working correctly with the size of your tires.

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