A vehicle's gear ratio is the speed of rotation of the propeller shaft (or drive shaft) in relation to the rear wheels. In most cases, the engine and transmission spin the propeller shaft much faster than needed, so the vehicle's differential assembly slows down that motion to a more manageable speed by way of ring and pinion gears. The most accurate way to determine a vehicle's gear ratio is to count the teeth on these gears, however, the differential would have to be disassembled to accomplish this. There is a must easier and quicker way to determine your gear ratio by spinning one rear wheel.
Park the vehicle on a firm, level surface and place wheel chocks under the front wheels of the vehicle to prevent any unwanted movement. Raise the rear of the vehicle with a hydraulic jack and support the axle housing on each side of the vehicle with sturdy jack stands. Lower the hydraulic jack.
Crawl under the vehicle and mark a horizontal line about two inches long on the propeller shaft with a piece of chalk.
Mark the bottom of one of the rear tires at the 6 o'clock position with a piece of chalk.
Use the chalk mark on the rear tire as a reference and begin rotating the tire by hand from a position that allows you to see the propeller shaft.
Turn the rear tire one complete revolution while simultaneously counting the number of revolutions of the propeller shaft indicated by the horizontal chalk mark. Be as accurate as possible.
Determine your gear ratio, which is the number of propeller shaft revolutions that were made in relation to one tire revolution. For instance, if your propeller shaft rotated three and one half times in relation to one tire revolution, your gear ratio is 3.50.
Use the hydraulic jack to raise the vehicle high enough to remove the jack stands from under the axle housing. Lower the hydraulic jack and remove it from under the vehicle. Remove the wheel chocks.
