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How can i determine the RPM of the engine for a given rear axle ratio?

This formula is from WT March 2011 in the Co-Operator and can give you RPM of the engine or The rear end ratio or what size a tire is needed for a given RPM.

RPM = (MPR x Gear ratio x 336 / Tire Diameter)

RPM = engine revaluations per minute
MPR = Miles per hour

RPM = 50 x 6.66 x 336 / 37.5"
RPM = 2983

This is a truck at 50 miles per hour with rear gears of 6.66 and tire size of 37.5 inches to get a RPM of 2983