What’s Your Gear Ratio
| 30 April 2008
Whether from curiosity or an actual need, there are times when you’d like to know the rear gear ratio in your Mustang. If the tag is missing, you have to calculate the ratio. You’re probably familiar with a procedure that will give you the approximate answer. All you need to do is count the number of revolutions that the driveshaft makes each time the rear wheels make one revolution. If the answer is 3½ for example, then the ratio is 3.50:1. Rear gear ratios are always expressed as the number of pinion gear revolutions (3.5 in the example) that it takes to make the ring gear go around one time. The pinion gear is connected to the driveshaft and the ring gear is connected to the rear wheels via the axles so revolutions of those parts can be counted to determine the ratio.
The problem is that unless you can measure exactly one revolution of the wheel and also the exact number of full revolutions plus fractional revolutions at the driveshaft, your answer has too much guess work in it. An approximate ratio may be close enough for your needs but if you’re looking for better accuracy, there are a couple of ways to get it. You can disassemble the differential and count the number of teeth on the ring and pinion gears or you can fine tune the way you measure revolutions at the wheel and the driveshaft.
If you already plan to disassemble the differential, then counting teeth will give you the absolute exact ratio, correct to several decimal places. Just divide the number of teeth on the pinion gear into the number of teeth on the ring gear. If your answer is 3.5, then the ratio is 3.50:1, just like the example above. But if you hadn’t planned to disassemble the differential, you probably don’t want to take it apart just to count teeth. So you need a refined method for counting revolutions. There is one and when you boil it down, it’s quite simple. It won’t give you the exact answer to several decimal places like counting teeth will but for many purposes it’ll be plenty close enough.
First, you need to find a board; one that has a length of something like two to four feet will work. Four or five inches from one end of the board, drive a nail through it until the nailhead is flush with the surface. The nail should be long enough so that the point sticks up two or three inches above the other surface. Second, you’ll need about 12 feet of strong twine. The twine should have a small diameter and it should be resistant to stretching. Next round up your jack, a couple of jack stands, a tape measure, some tape (duct tape is good or electrical tape will also work), materials to clean a small area of the driveshaft, a marker that you can use to put a line on a tire, and another marker that you can use to mark the twine—the finer the marking tips, the better. Finally, you’ll need help so find a helper. Come to think of it, maybe you should find the helper first because without assistance, it will be very difficult to do the job, probably making it unnecessary to collect all of the other stuff.
Jack the rear tires one or two inches off the ground and support the car with the jack stands. Crawl under the car and take the tape measure with you along with the tape, the twine, and the cleaning materials. Clean an area around the circumference of the driveshaft at an accessible location and then securely tape one end of the twine to the just-cleaned location. The piece of tape should be roughly parallel to the length of the driveshaft and the twine should be perpendicular to it. Now take your tape measure and wrap it tightly around the shaft right next to the twine. Measure the driveshaft’s circumference as accurately as possible and ask your helper to record the measurement.
With your helper rotating a rear wheel, wind the twine around the driveshaft at least six or seven times. If your rear gear is an “open” design, securely block one tire so that wheel won’t turn when the other wheel is rotated. (Don’t block the tire if the gear is a limited slip or a locker.) Take care to prevent the twine from overlapping itself but keep each wrap parallel to, and resting against, its neighbor. When you’ve finished wrapping the twine, ask your helper to keep the wheel from turning while you crawl out from under the car with the loose end of the twine in your hand. Keep enough tension on it to make sure that it doesn’t throw any slack around the driveshaft.
Ask your helper to place the board with the nail in it under the tire so that the nail point is close to the sidewall at the 6:00 o’clock position. While he holds the wheel from turning, you need to find a fixed location along the rocker panel or slightly under the car where the twine is resting if you tension it up against the floor pan. The pinch weld on first generation Mustangs is a good place. With your helper still holding the tire securely, have him put a line on its sidewall directly above the nail point. If you don’t want to put a mark on the tire, put a piece of masking tape on the sidewall and then put the mark on the tape. With the mark on the tire lined up directly above the nail point, pull the twine up against the pinch weld or other reference point that you chose. Mark the twine right at that spot. Mark the reference point too because you’ll need to come back to that exact spot later.
Keep tension on the twine and have your helper rotate the wheel exactly one revolution in the direction opposite to the one used when the twine was wrapped around the driveshaft. When the wheel has completed the revolution you will have pulled some of the twine from under the car and the sidewall mark will be located directly above the nail point again. If your rear gear is a limited slip or locker design, you’re ready to hold light tension on the twine and put another mark on it using the same reference point that you used before the wheel was rotated. If you’re working with an open gear don’t mark the twine yet. Instead, have your helper securely wedge the tire to keep the wheel from moving and then have him take the board with the nail in it to the opposite side of the car.
At the other wheel, your helper can remove the block that was placed there earlier. Next he can put the board with the nail in it under the tire so that the nail point is close to the sidewall just as he did on the first side. Follow the same procedure for marking the tire just above the nail point. Finally have him rotate the tire one full revolution while you continue to pull the twine from under the car using light tension. When the revolution is completed, have him hold the sidewall mark directly above the nail point while you hold tension on the twine and mark it where it contacts your reference point.
Now that the twine has two marks on it, you can calculate the gear ratio. Remove the tape from the driveshaft and pull the twine from under the car. Measure the distance between the two marks and divide the circumference of the driveshaft into that length. For example, the circumference might have measured 9⅜ inches (almost exactly a 3” driveshaft) and the distance between the marks on the twine might have measured 3213/16 inches. Dividing 9⅜ into 3213/16 gives you an answer of 3.5 for a ratio of 3.50:1, just like our earlier example.
That’s it. It’s a relatively easy procedure and it can be quite accurate, if done carefully. You won’t be able to tell the difference between a 3.89 and a 3.91, for example, but it won’t be hard to distinguish among gears that have more of a spread, such as a 3.27 vs. a 3.55 vs. a 3.73.