How To Determine A Mustang's Rear Axle Ratio

Determining Your Mustang's Gears

The first option will be going off of your factory specs for the model you own. We would like to note that this is not always 100% accurate, though. As many of us know, we all get the itch and modify our Mustangs. If you are not the vehicle’s original owner, someone could have made a gear change somewhere down the line. To find out more info on what gear your Mustang had from the factory, as well as everything you need to know about the rear end in general, check out our Rear End Gear Ratio and Axle Guide.

1. Start by safely raising the rear of the car via a lift or jacks and jack stands.


2. Rotate one of the wheels until you remove any residual slack within the differential.


3. Using some low-tack painter’s tape, tear off two small sections for the tire and driveshaft. Place one on the body of the driveshaft and then one on the outside edge mark on the tire. Place these both in the 6 o’clock position. *Please Note: On a vehicle that is an open diff. you would add tape to both tires and make certain that they both turn one full turn. Any movement of the spider gears would throw off the results*.


4. Rotate the tire counterclockwise and monitor the mark you have on the driveshaft. Next, you will begin by counting the number of driveshaft rotations until you have completed one revolution of the tire back to its original location.


5. Once you have this counted, this will allow you to know what gear set you may have. If the driveshaft turned roughly three and three-quarters of a turn, you would be right at a 3.73 gear ratio. If it was just over four turns, you should be at a 4.10 ratio and so on down the line. We would like to note here that some ratios are very close to each other. Just make sure to use your best judgment with your calculations. Let’s take, for example, a 4.10 vs. a 4.30 gear. If the tape mark is closer to the 6 o’clock position than the 12 o’clock position, then your gear would be a 4.10 vs. a 4.30.

With this information at your disposal, determining your rear-end gears should be a breeze! Whether you have a 79 Pace Car or a brand new GT500, this will take out all guesswork. For more informative tech articles and Mustang content, keep it here with the real enthusiasts at LMR.com.