GM Cutlass RWD 1970-1987 Repair Guide

Determining Axle Ratio


Determining the axle ratio of any given axle can be a very useful tool to the contemporary vehicle owner. Axle ratios are a major factor in a vehicle's fuel mileage, so the vehicle buyer of today should know both what he or she is looking for, and what the salesperson is talking about. Knowledge of axle ratios is also valuable to the owner/mechanic who is shopping through salvage yards for a used axle, who is repairing his or her own rear axle, or who is changing rear axle ratios by changing rear axles.

The rear axle ratio is said to have a certain ratio, say 4.11. It is called a 4.11 rear although the 4.11 actually means 4.11 to 1 (4.11:1). This means that the driveshaft will turn 4.11 times for every turn of the rear wheels. The number 4.11 is determined by dividing the number of teeth on the pinion gear into the number of teeth on the ring gear. In the case of a 4.11 rear, there could be 9 teeth on the pinion and 37 teeth on the ring gear (37/ 9 = 4.11). This provides a sure way, although troublesome, of determining your rear axle's ratio. The axle must be drained and the rear cover removed to do this, and then the teeth counted.

A much easier method is to jack up the vehicle and safely support it with jackstands, so BOTH rear wheels are off the ground. Block the front wheels, set the parking brake and put the transmission in Neutral. Make a chalk mark on the rear wheel and the driveshaft. Turn the rear wheel one complete revolution and count the number of turns that the driveshaft makes (having an assistant here to count one or the other is helpful). The number of turns the driveshaft makes in one complete revolution of the rear wheel is an approximation of the rear axle ratio.