differential question

[mikebloco]

I have an 91 325i auto car(yeah I know it sucks) and I crawled under it to see the diff code and it says 4:10 but when I'm cruising at 70 I'm turning 2,600 rpms. That makes no sense. Can someone elaborate?

[jeffnhiscars]

Switched tag or switch gears. With stock tires I'd guess 3.46 since IIRC my 3.73 turns 3000 at that speed

[mikebloco]

I'm rockin 16's

[jeffnhiscars]

What size tires

[mikebloco]

205/50/16

[jeffnhiscars]

Diameter is 24.1" which is within 3% of stock

[whodwho]

Autos came with a 410 to match the auto output ratio. When/if you switch to a manual trans it will r3v higher at the same speeds

[mikebloco]

Thanks. The rpms will drop a lot when I go manual because I'm going with an R154 and 3.73. Try and get some more mileage out of her.............and no I'm not mating an m20 to an R154 >;)

[jeffnhiscars]

Out of 5 e30s with auto trans the 89 cabrio I sold may have had a 4.10. The 3 87s we still have are all 3.73 and the 84 318i I sold which was born w an auto had a 3.64

[whodwho]

From RealOEM looks like ETA based autos had 3.73s which makes sense as they always had higher numbered diffs.

[jeffnhiscars]

I Misspoke as our ETA has a 2.93S. The other 2 are "i" cars w 3.73s.

[mikebloco]

I would kill for 3.73. Soon. Once all the suspensions done I'll do big break upgrade and diff.