2.93 rear diff in auto 323i

[shftr8]

So I have a 98 323 automatic coupe. If I remember correctly they have a 3.91. Its just my daily commute car and my commute is changing to 95% highway. would it be worth swapping in a 2.93 diff from a manual car? It should drop my cruising RPM significantly. Im afraid the car will constantly want to drop down a gear to pick up rpm. has anyone done this before that can chime in??

[gdavid]

I haven't done it but have considered it before as I have a long commute with an auto and I have access to an open 2.93 diff. Worse case it would drop out of overdrive and into drive which is a 1:1 ratio just like 5th in either of the manual transmissions. It would be pretty sluggish at initial acceleration from a stop and arguably you might as well just get a 318 instead. 318ti's are fun to drive, 300 lbs lighter and would provide better mileage if that's what your after.

[spyderg0d]

I wouldnt. It might drop rpm a bit. But it wont help mileage much. It might actually hurt it being more out of the power band than normal. Unless you drove faster ofcoarse. Haha.

[shftr8]

I live less then a half mile from the interstate and work less than half mile off and I drive about 80 usually. Usually around 3000 RPM at 80 and I found a calculator that says I should be around 2200 RPM with a 2.93. I can get a 2.93 rear diff for free and this is just a commuter car for me as I have another vehicle

[Moron95M3]

I think our M50's are designed to be more efficient up near 3k rpm vs. 2200 rpm...not 100% sure but from my experience I used to get 30mpg at 3k rpm highway cruising (S50).

[jmo69]

I think our M50's are designed to be more efficient up near 3k rpm vs. 2200 rpm...not 100% sure but from my experience I used to get 30mpg at 3k rpm highway cruising (S50).

98 323 isn't an m50. I've made 30+ with my 328is manual at highway speeds but not 80 mph.

[spyderg0d]

Not sure if your 4 bamger car would be able to get amewhere with a 2.93.. Stop at a stop light and its done for. Youd be getting to 10 mph by the next mile marker . lol

[gdavid]

Not sure if your 4 bamger car would be able to get amewhere with a 2.93.. Stop at a stop light and its done for. Youd be getting to 10 mph by the next mile marker . lol

Not sure if you are talking about another car but the OP should have the M52b25 in his 323 which has a lower peak HP and torque than the M50 but it should have better torque at lower rpm than the 325. It would probably feel a lot more like an auto 318 until you get through first gear but aside from that, it would probably accelerate pretty similarly. It would probably just move the shift points to the right, these automatic transmissions don't pay any attention to the car's speed. The performance would probably depend a lot on the stall speed of the torque converter.

That said, the overall mileage probably won't improve, I'm sure BMW played around with all of these options when they set it up but it may be a bit quieter and smoother at highway speeds with the driveshaft turning about 25% slower (assuming it will stay in overdrive). It would probably work fine in Florida where the OP is. I am curious about the results but no where near curious enought to swap a diff (most likely twice).