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.
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 2004r 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.
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