A number of factors must be considered when selecting a rear-end 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 rear-end gear ratio can be selected using the following simple formulas whether using a direct-drive (1to1) or overdrive (usually 30%) transmission.
FIGURING TIRE DIAMETER (Height)
FORMULA: Tire Size / 25.4 x Aspect Ratio x 2 + Wheel Diameter = Tire Diameter in Inches (Height)
EXAMPLE (285x70R15): 285 / 25.4 = 11.22 x .70 = 7.8540 x 2 = 15.70 + 15 = 30.70
FIGURING TIRE CIRCUMFERENCE
FORMULA: Pi (3.14159) x Tire Diameter = Circumference in Inches
EXAMPLE (285x70R15): 3.14159 x 30.70 = 96.446813
CONVERTING TIRE CIRCUMFERENCE TO REVOLUTIONS PER MILE
FORMULA: Tire Circumference in Inches / 12 = Tire Circumference in Feet
FORMULA: Number of Feet in a Mile / Tire Circumference in Feet = Tire Revolutions Per Mile
EXAMPLE (285x70R15): 96.446813 / 12 = 8.0372344
EXAMPLE (285x70R15): 5280 Feet / 8.0372344 = 656.94239
Below is a simpler formula used to figure tire revolutions per mile with an acceptable variation from the more exacting formula using tire circumference to figure tire revolutions per mile.
FIGURING TIRE REVOLUTIONS PER MILE
FORMULA: 20168 / Tire Diameter = Tire Revolutions Per Mile
EXAMPLE (285x70R15): 20168 / 30.70 = 656.93811
FIGURING REAREND GEAR RATIO (Ring & Pinion)
FORMULA: Engine RPM @ Cruise / Tire Revolutions Per Mile = Rear-end Gear Ratio
EXAMPLE Without Overdrive: 1800 RPM / 656.93811 = 2.73:1
EXAMPLE Without Overdrive: 2000 RPM / 656.93811 = 3.04:1
EXAMPLE With Overdrive (30%): 1800 RPM / 656.93811 = 2.73:1 x 130 = 3.54:1
EXAMPLE With Overdrive (30%): 2000 RPM / 656.93811 = 3.04:1 x 130 = 3.95:1
Rear-End / Differential Gear Ratio Calculator
Enter the maximum potential safe engine RPM. Enter the tire size diameter and the target vehicle speed in MPH. Click “Calculate Gear Ratio”. The value will be returned in estimated differential gear ratio, based on the entered values…
