M42 Differential choice?

[91318is]

I had a 3.73 open before and it was much more civil on the freeway. Currently have a 4.10 which is great for streets but rocking 4K RPM at 80 mph isn’t very great. Anyone use a 3.25 or anything around there? Hoping to get a 75 mph cruiser at 3k RPM. Pretty sure that would do it. Thoughts?

side note, I am open to upgrading to a LSD

[fresh_TD]

Pmed. 3.64, 3.73 ratios, are perfect for the M42.

[roguetoaster]

For several years before I sold my 318i it had a 4.44 in it. The car felt quick, still managed just under 120mph, and returned 27 or so MPG in mostly highway driving. Positives, good for burnouts, good for passing in 5th gear, excellent for flying on the highway, also great for creeping along in traffic. Cons, the high RPM on the highway can get tiring, but it also encourages you to keep your actual speed down.

For one 1200 mile trip I had a 3.15 in the same car, and it was very peaceful on the highway, but passing was sometimes a two gear drop, while getting going wasn't bad as you might think since you could stay in 1st gear between lights in town. Pretty sure the car averaged about 30 in mostly highway driving, and the E30 almost hit 130mph, but was probably a bit weighed down.

Basically, as said above, somewhere in the middle between 4.44 and 3.15 is probably best.

[varg]

I have a 3.25 in my E30, and it's great, but would not have done that when I had a 4cyl. You should be doing around 85mph at 4,000rpm in 5th with a 4.10 depending on tire size, I agree with fresh_TD on gear ratios here.