Typically , the best cruising range for an engine is from 2,000 to 2,400 RPM. Here are some formulas to help you calculate the best gearing for your car.
mph = (rpm x tire diameter) ÷ (gear ratio x 336)
rpm = (mph x gear ratio) x 336 ÷ tire diameter
Gear ratio = (rpm x tire diameter) ÷ (mph x 336)
Tire diameter = (mph x gear ratio x 336 ÷ rpm)