Reference Manual

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Chapter 3: Axles: RPM calculation

Changing a rear axle gear ratio will have an effect on engine RPM for any given speed. The taller (numerically lower) ratio would lower engine RPM, while a stiffer (numerically higher) ratio would raise engine RPM for any given speed. This can be calculated using the following formula or variations of it:

(axle_ratio * transmission_ratio) * tire_rev_per_mile = engine_rev_per_mile

engine_rev_per_mile also happens to be the same as rpm@60 mph.

The formula above will result in a very accurate RPM vs. MPH. However, you might only be be looking for an idea of what engine RPM you would see with a given axle ratio. The calculator below can be used to obtain an idea of how a given ratio will effect engine RPM.

NOTE: Javascript will have to enabled in order to use this calculator.

Calculator Notes:

The result generated by this calculator is highly dependent on an accurate tire diameter. Actual tire diameter can vary quite a bit from brand to brand and even amongst lines within a brand. The tire calculator should be close, but don't depend on it. Even so, the results should be within 100 - 200 rpm of true engine RPM. For best results, measure from the ground to the center of the tire and multiply by 2 for the tire diameter, then enter that number.

The default transmission ratio is for the 200-4r in OD. Enter 1 for a 3 speed transmission. The ratios of other gears could also be entered to obtain a RPM in gear at speed calculation. Another possible use is comparing how different tire sizes would effect engine RPM at a given speed.