What's the most reasonable differential

[pob000]

for my 99 M3. I can get my hands on a 3.91 for a pretty decent price. Since I'm getting my top end rebuilt, I figure I drop in some lower gears for off the line acceleration. Tell me what you think?:help

[saint166]

i have a 3:91 diffy and with a 5 speed on the highway the car reved kinda high . i now have a 6 speed so for highway cruising its great . i do love the acceleration i get from the 3:91 diff

[sikdogg]

With that low of a gear ratio, i would think that 1st gear would be almost useless.

[desynch]

I'd look at getting a 3.38 or 3.46 if you don't have many other mods.

3.91 is way too excessive. I've heard others that used it makes 1st useless.

[RRSperry]

3.91:1 in a 5 speed car...lol

4250 rpm at 80mph, almost no traction in 1st. Eh no thanks...

For a 5 speed car that sees any time on the highway, a 3.46 is the shortest raitio I would consider. for a 6 speed car 3.64. I got a deal on a 3.38 about 7 years ago, and am very happy with it. (7 YEARS??? arghhhhh)

[pbonsalb]

A 15-20 year old 3.91 from a 325 with sports package and automatic is probably the cheapest. A 3.38 from a 96-99 M3 automatic is next cheapest and is a pretty good compromise. It will be only a mild on a 99 that came with a 3.23, but is enough to feel. I ran one for a year in my 99M3 before moving on to a 3.64. The 3.64 is low for a car that sees a lot of highway use. I don't take many long trips so the 3.64 works for me. A 6 speed would be great but I am not sure I want to spend another $2k.

[libravcs]

4250 at just 80mph?

Thats insane.

[Crazy 13immer]

I drove a 3.91 in a 5-sp because I was considering changing my diff. But the 3.91 is too much if you do any highway driving. It's basically like dropping down one gear.

I would say to go with a 3.38 but I think they are kinda $$.

[Brent 930]

You not only lose your 1st gear with a 3.91 but also 5th for cruise.

[desynch]

Haha, if you like riding on 4th gear the whole time...wouldn't be fun for gas mileage for you.

[EastBayBimmer]

3.38 is a good compromise

[///Montizzle]

3.64 is the best all around diff

[pbonsalb]

My 3.64 is like a 3.58 with my larger than stock 255/40/17 tires. But it is still a bit much for a high speed long distance highway cruiser.

[Analog]

I LOVE my 3.38. It's damn near perfect for DD and track usage mix, IMO.

[bmdubbayoo]

I LOVE my 3.38. It's damn near perfect for DD and track usage mix, IMO.

ditto