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
(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.