A number of factors must be considered when selecting a rearend gear ratio (ring & pinion set). Tire diameter, transmission choice (with or without overdrive) and desired highway cruising speed.
Tire diameter is the first consideration because styling and available space within the wheel opening dictate the size of tire / wheel combination that will be chosen for the car. The choice of a direct-drive (1to1) or overdrive (usually 30%) transmission will determine driveshaft RPM at cruising speed. A highway cruising speed of 60—65 MPH is considered a good target speed
Engine RPM at cruising speed should be maintained between 1800 to 2100 RPM. Lower RPM at cruising speed will produce a vibration that is often mistaken for driveshaft imbalance. This vibration is actually caused in a typical V8 engine by acceleration of the piston on a downward power stroke that is only cancelled out by other cylinders doing the same in rapid succession. This only occurs when the engine is revving in excess of 1800 RPM. This vibration isn’t noticeable when the engine is idling or accelerating through the gears but is often misdiagnosed as a driveshaft out of balance of out of phase.
The proper rearend gear ratio can be selected using the following simple formulas whether using a direct-drive (1to1) or overdrive (usually 30%) transmission.