You just bought your latest hot rod, but in true car-guy fashion, you think an upgrade is in order. You have decided that a rearend gear swap is the first place you want to go. Before you can decide if an upgrade is in order, don’t you think you should first find out what gear ratio your car already has in the rear?
How to determine what gear ratio is under a car is a question that I get asked quite often. I am surprised at the frequency of the question, since finding the answer is a really simple task. While you can pull the rearend cover and look for the ratio numbers on the ring gear, there is an easier way. In fact, it only requires you have a floor jack and a couple of jackstands. To help you guys make gear identification easy, I decided to put together a small outline of how you can find out what gears you actually have without opening the rearend.
Knowing your rearend’s gear ratio is crucial when you are considering a transmission upgrade, needing to calibrate the speedometer gear in your transmission, selecting a torque converter, or even getting professional help with camshaft selection.
Before you can begin identifying the gear ratio, you will also need to know if the rearend 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 jackstands 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 jackstands 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 jackstands and put one tire back on the ground.
While doing this test, some guys like to use soapstone or chalk 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 my reference. If using a paint marker, make a large, visible dot on both the inside of the tire and on the driveshaft.
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. Let’s say 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. Again, 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.
Hopefully, this little tutorial will give some knowledge that every car guy should know and makes any decisions about changing gears a little less daunting.