Welcome Guest [Log In] [Register]
Viewing Single Post From: How to Determine Your Transmission Final Drive Ratio - In the Car or on the Bench

Here's the procedure for determining the final drive ratio of a transmission while it's still in the car.

See my post of April 7, 2011 (page 2 of this thread) for doing it to a transmission on the bench.

Also, pacapo deserves a lot of credit for this whole concept because it was his original post that put me onto the idea of turning the engine over so many turns and counting wheel revolutions. I neglected to mention him in my original post because I couldn't remember where I saw it in the first place. It was his post that started all this (and he never even mentioned it when he complimented me on the original post).

While getting my 94 XFI parts car ready for the tranny and camshaft swap I discovered some yellow junkyard paint on the valve cover. This, of course made me think it might not be the original motor. That made me start worrying that if the engine has been changed, maybe the transmission has too.

I pulled the valve cover and measured the lobes on the cam and found, much to my disappointment, that several measured 1.592" which means it isn't an XFI cam. (XFI's = 1.5602 to 1.5665, standard 3 cylinders = 1.5911 to 1.5974).

Now I was really worried about the transmission! How can I tell which one it is?

Although I found a few guides on the forum I still wasn't sure how to tell. So, using gear ratios and final drive ratios I calculated how many turns an elevated wheel would make per revolution of the engine in various gears. Although I was really only concerned about my transmission I figured I might as well calculate all four possible final drive ratios for the benefit of others. I finally selected the 5th gear numbers to use since they result in the greatest wheel angular difference per engine revolution.

I then performed the procedure outlined below on three different 5 speed Metros that I own (yeah three, ask my wife!): my '95 3 cylinder; the '94 XFI parts car; and my '93 3 cylinder base model.

And the results were just as I hoped and expected (and, hooray, I learned that I do have an XFI transmission!).

Here's the procedure to use:

- Block one or both back wheels
- Key off, put the car in 5th gear, pop the hood release
- Elevate either front wheel of the car (the right wheel is easiest - see below). The other front wheel must remain stationary.
- Open the hood and rotate the elevated tire until the timing mark on the crankshaft pulley is lined up with the indicator (pick either end, or whatever you can see best)
- Put a mark on the tire at BDC (bottom dead center) (it's OK if it's slightly off - don't go crazy getting it exact). Mark the ground too, for reference.
- Watching the crankshaft pulley, rotate the tire CLOCKWISE until the ENGINE has made exactly three revolutions
- Observe the location of the mark on the tire to determine which final drive ratio you have

4 o'clock = 4.39
5 o'clock = 4.10
7 o'clock = 3.79
9 o'clock = 3.52

Below are pictures of the 3 different cars and three final drive ratios. I actually performed the above procedure on all three cars and did not fudge the results. These are honest wheel locations after three engine revolutions. Because I wanted to have pictures of all 4 ratios, I simulated what the 3.52 would look like by turning the XFI wheel to the 9 o'clock position and taking a picture. I'm sure it's correct though.

I elevated the right front wheel on all the cars beacause I was working alone and you can see the crankshaft pulley while rotating the right wheel but you can't when rotating the left wheel. If you must use the left wheel remember to turn it clockwise. Also the position of the BDC mark will appear exactly as is does in the photos: 4 o'clock, 5 o'clock, etc, not reversed.

This is where you start.

Posted Image

Rotate the engine exactly 3 revolutions (turning the tire clockwise to rotate it) and look to see where the BDC mark ends up.

Below is a '95 three cylinder, 4.39 ratio

Posted Image

Below is a '93 three cylinder, 4.10 ratio

Posted Image

Below is a '94 XFI, 3.79 ratio

Posted Image

And lastly, here is what a 3.52 ratio would look like (I faked this picture because I don't have a 3.52 - it's accurate though)

Posted Image

If you need to check out a transmission and can't remember these numbers or photos remember this: the "good" ones (3.52 & 3.79) produce MORE than 2 wheel revolutions and the "bad" ones (4.10 & 4.39) produce LESS than two revolutions. And the extreme ratios (4.39 and 3.52) are both about 90 degrees away from two wheel revolutions.

Edited by Wobblybob, Apr 29 2011, 09:04 PM.
Offline Profile Quote Post
How to Determine Your Transmission Final Drive Ratio - In the Car or on the Bench · Transmission/Clutch/Axles