What diff for 60mph in 2nd

[Jean]

Guys, to make sure my 2nd gear maxes out at about 60mph or so what diff ratio should I look for if I'll use an e36 5spd with an s50 engine ?

I am building a car for next seasons autocross and don't want to end up with something that tops out at 40mph in 2nd or at 80mph.

Thanks!

[backtrail69]

Make sure to use the ZF 5sp, the Getrag 250 is garbage. I know the OE m3s came with 3.23 rear ends. So anything there or lower should get you there.

[PiercedE30]

3.25 will put you at 60 MPH at 7018 RPMs in 2nd gear with the ZF tranny (using stock height tire of 23.9 in tall).

[Jean]

ZF it is :) What would 3.7 yield?

[PiercedE30]

With a 3.73 it would put you right around 51 MPH.

[Jean]

Thank you! Just out of curiousity, how much do these numbers change when e30 5spd is used instead of ZF?

I'll look for a 3.25 and use my current 3.7 to have two different diffs to play with depending on layout or whatnot.

[PiercedE30]

3.25 - RPM - MPH
G260 - 7000 - 70
ZF - 7000 - 62

3.73 - RPM - MPH
G260 - 7000 - 61
ZF - 7000 - 54

This is using 2nd gear ratios of 2.49 for the ZF and 2.20 for the G260.

[Jean]

Appreciate it!

[Aptyp]

what are the rest of the ratios for ZF?