Rear Gears

[threlevenguy]

Hey I've got an M50 car with what I believe is a 3.73 rear diff in the back. At about 60-65 mph I'm hovering over 3300-3500 RPM and as you 24v guys know, that's dangerously close in the torquey highway pull department :twisted:. It kills my highway mileage and just isn't fun long distance.This fall I'm going to be making a lot of long distance trips on the highway so I was wondering if anyone could give me some pointers on what diff I need to get and maybe some recommendations on the correct gearing to where I would be getting better gas on the highway by keeping the tach needle as far down as possible when cruising.
Thanks,
Matt

[nando]

I would be looking for a 3.55 (if there is such a thing for E30s)
. 3300-3500 is quite high for 65mph, I turn 3500 at 75mph which is fine (and great for passing).

lots of guys go with 3.25s or lower, which I don't really understand, but then I'm a big fan of 4.10s..

[threlevenguy]

I'm a noob with gearing, is 4.10 good for the highway? If the number is higher, does it mean the final gear ends farther?

[z31maniac]

Which trans are you using? E30 or E36?

[DragonOrta]

I'm a noob with gearing, is 4.10 good for the highway? If the number is higher, does it mean the final gear ends farther?

Other way around. With a 4.10, your RPMs will be higher than with the 3.73 in any given gear.

[Rigmaster]

3.25 is about the only commonly available diff that will bolt in (found in several different E28's). Most people say a 3.46 or 3.64 is ideal, but they're really hard to find in an E30 case.

[achillies]

Aren't most 84-85 318i cars 3.64? THere should be lots of them. But small case no lsd.

[threlevenguy]

Which trans are you using? E30 or E36?

Oops sorry, its the stock tranny from the m42.

[threlevenguy]

So a 3.25 diff from an e28 would bolt right in?

[threlevenguy]

Anyone else chime in?

[FredK]

So a 3.25 diff from an e28 would bolt right in?

Yes, as long as you swap the rear cover to a medium case E30 diff rear cover, and swap the output flanges of the diff to the medium case E30 flanges.

The swap should be no problem if you have a medium case 3.73. If you have a small case 4.10 as equipped in the 318is, you will have to find a medium case E30 diff for the covers and output flanges.

[Wh33lhop]

Yes, as long as you swap the rear cover to a medium case E30 diff rear cover, and swap the output flanges of the diff to the medium case E30 flanges.

The swap should be no problem if you have a medium case 3.73. If you have a small case 4.10 as equipped in the 318is, you will have to find a medium case E30 diff for the covers and output flanges.

Yep. My 3.73LSD came with E28 diff cover and axle stubs, it only took a few minutes to swap them.

If you're running an E36 transmission (direct drive/non-overdrive) then that would explain the excessively high revs; their fifth gear is as short as our fourth (1:1), you'd be turning over 4300rpm at 80 if you're turning over 3500 at 65, so that sounds about right.

If you're running an E36 transmission, I'd go down to an E36 ratio, a 2.93 or 3.25. Our rolling diameter is a little smaller so you will still be effectively geared a little shorter than the E36 even if you get a 2.93.

[threlevenguy]

I appreciate you all's help..
Would this be a good application for me?

http://cgi.ebay.com/ebaymotors/BMW-525is-3-25-1-Limited-Slip-Differential-E28-E30-Z3_W0QQitemZ320409360578QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item4a99e2d8c2&_trksid=p4506.c0.m245

or

http://cgi.ebay.com/ebaymotors/BMW-E30-LIMITED-SLIP-DIFFERENTIAL-2-93-LSD-325I-325IS_W0QQitemZ250479420565QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item3a51bca495&_trksid=p4506.c0.m245

[Wh33lhop]

Either of those would be fine. The first one has E28 diff cover and axle stubs, the second one is set to drop into an E30 and will reduce your cruising rpm a little more.

[threlevenguy]

So say at 65-70 mph, what would you guess I'd be cruising at (RPM-wise)?