HOW TO DETERMINE YOUR GEAR RATIO
Knowing your rear end’s gear ratio is crucial when you are considering a transmission upgrade, needing to calibrate the speedometer gear in your transmission, selecting the proper gear sets in a transmission, or even getting professional help with a new axle ratio for your application.
There are ways to determine your gear ratio without having to open your differential.
The differential also called a pumpkin, is a mess if you open it up.
The gear oil is smelly, it takes a lot of time, and you want to avoid opening it up if possible.
So, there are some places you can check and things you can do to determine your gear ratio without going through all that.
Check the Differential Cover
There are two other places that you can check…On the Axle and the differential cover itself
The axle may have a sticker, and on the differential cover, you might have a small metal tag that’s sticking out that will have the gear ratio stamped on it.
Do a Driveshaft Turn Test.
Before you can begin identifying the gear ratio, you will also need to know if the rear end is a “posi” or open differential.
This is required because identifying gear ratios is a little different for each unit.
To begin with, raise the rear of the car with a floor jack and place a good set of jack stands under the car on the frame or rear-axle housing.
Once supported, place the transmission in Neutral and turn one of the rear wheels.
- If the opposing wheel spins in the same direction, you have a posi – or limited-slip – differential.
If this is the case, you can leave both jack stands under the car.
- If you spin one of the rear wheels and the other one rotates in the opposite direction, you have an open differential.
In this case, you will need to remove one of the jack stands and put one tire back on the ground.
While doing this test, some guys like to use soapstone, chalk, or tape to put a mark on the tire and driveshaft to make counting the total revolutions easier. If you also want to do that, a modeler’s paint marker also works well. I typically focus on an already supplied marking of some sort on the tire (like a certain letter of the tire’s name) and one of the universal-joint bolts and use those as your reference. If using a paint marker, make a large, visible dot on both the inside of the tire and on the driveshaft.
(we used tape)
When you have your marks applied, if the car has a posi unit, both tires will be in the air.
Rotate the marked tire one revolution while counting how many times the driveshaft rotates.
eg, If your driveshaft made 3 3/4 revolutions, that would determine a 3.73 gear ratio.
If you get approximately 2 3/4 revolutions, you have a 2.76 gear ratio.
Like previously mentioned, if your car has an open differential (non-positraction), you will want to perform the check with one rear wheel on the ground. Again, mark the inside of the tire and a spot on the driveshaft with your paint marker. Now, rotate the suspended tire two revolutions and count the number of driveshaft turns. The reason to make two revolutions is the open differential actually rotates half as many times as a ‘posi,’ because of the design of the “open” gear operation.
if the driveshaft turned 3 3/4 revolutions for two rotations of the tire, you have a 3.73 gear. A driveshaft that rotates 2 3/4 times announces a 2.76 gear ratio.
Check the Ring Gear Stamping
If you have access to the differential and the cover is already removed look on the ring gear and turn it until you can see the stamping, and you’ll see the number of teeth.
That is the number of teeth for the ring gear and the pinion gear.
If you are not sure about your results when using the wheel rotation method, the number of teeth is stamped on the ring gear. The small number is the pinion-gear tooth count. The large number is the ring-gear tooth count. To find the gear ratio, divide the big number by the little number.
In the example to the left, 37 ÷ 12 = 3.083333. These are 3.08 gears.
Calculate the Ring & Pinion Teeth
If you can’t find a stamping or it is unreadable.
Then you can always count all the teeth on the pinion & ring gears.
Once you have those two numbers divide the number of teeth from the ring gear by the number of teeth that are on the pinion gear.
The answer will be your gear ratio.
Hopefully, we have provided you with some knowledge that every car guy should know & will help with any decisions about changing gears a little less challenging.
If you have any questions as always we are here to help