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
(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
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.
Calculating your axle gear ratio without opening your differential.
This can be done by jacking up, or both tires of one end of your vehicle off the ground. You have to note if one or both have been lifted as that affects the equation, but does not matter which method you choose. For me it is usually easier to just jack one wheel off the ground.
Mark the lifted tire with a line on the tire and the body at the same point. This can be done with either a white temporary marker, a piece of chalk, or a piece of masking tape.
Mark the prop-shaft also to a corresponding point on the axle housing.
With the differential or hub engaged, turn the tire 10 full revolutions past that mark on the body, whilst you have a buddy, count the number of time the mark on the prop shaft passes the corresponding mark on the axle. Write that down.
Times the number of revolutions of the prop shaft by two, because you only spun one tire, and divide that number by 10 because you spun it ten times to get a more accurate result. For example:
You spin the tire 10 times, and the prop shaft spins 23 and a half times past the mark on the axle housing, then equations will look like this.
23.5 x 2 = 47 /10 = 4.7
In this instance your diff ratio is 4.7, or the closest your vehicle make offers.
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