Which ratio diff do I have?

[mctenold]

I thought I saw this thread somewhere but I can't seem to find it now.

Basically I just bought a 1989 325i, and it has an LSD in it. Already did the test to make sure (lifted the rear, spun one wheel and the other one spun the same way).

But, it doesn't have that little metal tag under the bolt on the rear cover. Is there any other way to figure out what ratio diff I have?

If it helps, at 80mph I'm just a touch over 3,500 RPM. But man, it seems like I go through the gears fast.

Any help is appreciated.

Thanks guys!

[reelizmpro]

If it doesn't have a tag then you have to remove the diff cover and count the teeth on the ring gear and calculate it. Not worth the hassle unless ur changing the gasket anyway. Should have come with 3.73 if ur manual bit things often change over the years.

[mctenold]

The issue is that I bought a 3.73 LSD recently, and this white e30 I bought already had an LSD installed. I'd rather have the one with the lower ratio in the car because I do a lot of highway driving. But without doing a lot of work I'm trying to figure out what ratio is in the car now.

[frankie591]

jack both back wheels up, lie under the car with the trans in neutral. turn the driveshaft with your hands and count how many turns of the driveshaft turns the wheels 1 rotation. Either mark the wheel with chalk so you can see, or have a friend help you.

[mctenold]

jack both back wheels up, lie under the car with the trans in neutral. turn the driveshaft with your hands and count how many turns of the driveshaft turns the wheels 1 rotation. Either mark the wheel with chalk so you can see, or have a friend help you.

Maybe I'll try this.

Will I really be able to tell between say, a 3.73 and a 3.64 with this method, going by hand and eye?

[frankie591]

If you got the wheels to spin 10 times, you could tell the difference between 36 and 37 turns of the driveshaft. mark the driveshaft with chalk too so you can count the revs.