auto/5sp swap final drive ratio recomendation

[DaYouper]

completed auto to 5sp conversion on 81 633csi, I have only driven it around circle driveway so do not know how its gonna feel on the road but a question; It has oem diff, believe its 3:45 and only one rear wheel spins/drives (r/r) and that makes it useless to drive if any snow on ground/driveway. Could not even move in/out of garage if any amount of snow on driveway, car would just sit there and spin r/r trying to move forward inch by inch,,,not that I drive it in Winter but,,,anyway I have a LSD 3:90 was planning to install, had done that on couple other older BMW's (Bavaria, 5'ers) and really liked it in those models, yep loose a bit of top speed, but really liked accell, etc and with 5sp which is OD anyway thought this might work well.
thoughts,,,

[jps635]

Up to you really. I changed my 3.07 to 3.45 and wouldn't go with a lower diff ratio than that for my tastes

Have a play with this gearing calculator. http://www.unixnerd.co.uk/revs.html

[baders]

Depends on your driving requirements and vehicle specs. As I have 265/5 dog leg and S38B36 , and love highway/country driving, I chose to stay with the OEM 3.07. Loooooong legs

[86_6series]

I did a 5 speed swap with a 260 and kept the 3.45 from the auto.
For me it works great plenty of snap off the line and cruising @ 75 around
3000 ish RPM

[GRNSHRK]

I have a 5 speed OD gearbox in my '80 with the standard 3.45 rear axle ratio, but would love to go one step up to 3.64, if I could find one

Presumed that you have a sideloader diff

What model BMW had a 3.90 sideloader

For the record, at 3000 RPM in 5th gear, I'm registering 78 MPH

[bpoliakoff]

I have been rebuilding and selling diffs for a while and have found what I believe to be the definitive answer. 3:07 for a dogleg and 3:45 for an over drive box and of corse an LSD. For you guys with the later cars finding a lsd diff is not a problem as they are all over the place cheap. The killer is finding an early side loader diff for the pre 82 cars

[86_6series]

I stand corrected, GRNSHRK is absolutely spot on with MPH @ RPM
Don't remember what the hell I was looking at.

[LarryM]

I have a 3.91 diff in my car; I installed it when my car was still an automatic and left it in after my manual swap. First gear is very steep; you'll be shifting to 2nd before you're through an intersection. And at 80 mph the engine's turning about 3400 rpm so it's not something you'd want if you do a lot of long-distance driving. But other than that's it's all good; the car pulls much stronger in every gear and feels much more lively. Since I don't take the car on long trips, the highway revs don't bother me. If that's the kind of driving you plan to do, I recommend going with the 3.90.