There are ways to determine your gear ratio without having to open up your differential. The differential, also called a pumpkin, is a mess if you open it up. The gear oil stinky, it takes a lot of time, and you want to avoid opening it up, it if at all possible. But there are some places that you can check and things that you can do to determine your gear ratio without having to go there.
The first place to check is really easy. It’s most likely in your glovebox, or your paperwork when you bought your vehicle. It’s the window sticker, also called a Monroney. If you have a build sheet (if you had a custom-built from the manufacturer), you’ll also find it there.
As you can see right here, this stock 2005 F-150 had a 3.55 ratio limited-slip. Now I’ve changed it to 4.56 but that’s how it started its life.
Check Differential Cover
Now if that doesn’t pan out, there’s two other places that you can check… on the axle and the differential cover itself.
On the axle you might have a sticker, and on the differential cover you might have a little metal tag that’s sticking out. That will have the gear ratio stamped right on that.
Do a Driveshaft Turn Test
If you’re still not getting anywhere, jack up the rear end of your vehicle and mark your wheel and the driveshaft. I just used some blue tape. Turn your wheel one time. While you’re turning it one time, count the number of rotations on your drive shaft. That will determine your gear ratio.
In my case it rotated about four and a half times and my gear ratio is 4.56, so I know this test works spot-on.
Check Ring Gear Stamping
So you’re still not getting anywhere? Well, guess what? You’re going to have to crack that differential on open because you’re going to need to get to the ring gear and possibly take a look at the pinion gear as well.
If you can get to the ring gear, turn your wheel until you can see the stamping on the side of it. You’ll see the number of teeth. That’s the number of teeth for the ring gear and the pinion gear.
Calculate Ring & Pinion Teeth
You can’t find the stamping on the ring gear? Start counting! Count all the teeth on the pinion gear and the ring gear. Once you have those two numbers, divide the number of teeth that are on the ring gear by the number of teeth that are on the pinion gear and BAM–you have your gear ratio!
Where Else To Look
Here my last few bits of advice. First, if it’s a new vehicle don’t go ask your salesman. He’s not going to know these technical details and if he does give you an answer, it’s probably the wrong one… so just don’t even bother.
The other one is to check online forums for an answer. You might find somebody else that just bought that vehicle and they have the exact same question and they have the answer already. But, your vehicle might still be different.
The best thing to do is to use all those techniques and look at all those places that I just showed you and you’ll be able to determine the gear ratio for your vehicle.
If you have any other advice, please leave it in the comments section, below.