What’s up with my speed- e30 M50 ZF320

[WhiteE30ic]

Hi all- I can use some help on my 1985 E30 with M50 and ZF320. The car has a 3.15 medium case from a Z3.

I’m on 225/45 r15 tires and am verifying my speed via GPS.

I am nowhere near where I should be for speed. I’ve used a
couple of calculators and attached the diffs online version here. I should be seeing 87 mph at 4k RPM, but I’m seeing 71.

I was wondering if the 3.07’s I purchased were actually higher ratios so I swapped the 3.15 in with similar result (the 3.07 was 73 @ 4k rpm).

What gives? I know the ratios are basically math, so how can I be this far off?

[Panici]

The online calculators, they are set specifically for the ZF320's gear ratios?
5th in the ZF320 is 1:1, not overdrive like on the G260.

[WhiteE30ic]

Yeah- thanks. If you look at that attachment it has the ZF320 ratios on the top. It’s from diffsonline (he builds BMW stuff).

Also, overdrive would have the opposite effect, meaning revs would be too low. It’s almost like my 5th is a 1.24 you normally find in 4th.

[TobyB]

Maybe you have a 4- cylinder tach?

t

[WhiteE30ic]

Thanks for that- nope. I had the cluster rebuilt last year, and the revs are for sure accurate.

It is a super weird problem. If I can’t figure it out I can always throw a 2.65 in there…