differential ratio help

[nervousmini]

Hi there.

I have a 1984 BMW 533i with 420,000 miles on it and have been averaging about 25 mpg. I have been considering swapping out the differential for a better ratio but I was wondering how much of a gain it might make. Anybody better with a calculator than I am care to lend a hand?

Stock ratio = 3.25:1
alternate 1 = 2.93:1
alternate 2 = 2.79:1


I can get ahold of the 2.93 pretty easy, the 2.79 might take a bit of searching but they are out there. Any suggestions or thoughts on how much either might help my mpgs?

Thanks for the help.

[ex-x-fire]

The gain will depend on what type of driving you do most. If it's mostly hi-way driving, I say do it. If you do more city driving, then the gains might be great.
On my daily driver, an S-10 pick-up, w/ 3.08 axle, 4spd o/d trans, was getting approximately 25 mpg. At 70mph=2,000 rpms.
I added an add on overdrive to it. Mpg went close to 30mpg(not at 70mph). At 70 mph=1550rpms or so. Its been awhile since I've been to 70mph, I mostly drive 55mph at 1150-1200rpms. My total overdrive ratio is around 0.55, so my 3.08 ratio is nearly half in top gear.