Auto with 3:73 diff

[tinkerputzer]

I know 325i auto's normally come with a 4:10 diff. Would there be any down side of using a 3:73 other then a bit slower acceleration.

[Sagaris]

I can imagine that the ATF fluid temperature would get hotter if you run it hard and do a lot of stop and go, but as long as you arent a complete dope, I cant imagine it being a real problem since there are other cars with similar ZF automatics with more weight/longer gearing. I would get 195/60-14 series tires (assuming you are running stock 195/65-14's) to help even it out. I can run calculations for you if you give me your current tire size to tell you what the effective diff ratio would be and all that .

[tinkerputzer]

Tire size is stock along with the rest of the car. It is a daily driver that see's little abuse other than a bit of stop and go driving. I'd like to use the car for some long road trips and thought the longer gearing would be helpful.

[MR 325]

I've seen some autos that had the 3.73. My 89 was auto when I bought it with a 3.73 LSD and 245's and it was fine.

[tinkerputzer]

Thanks! That is good to know. Speaking of a 3:73 lsd, do you have one you're are willing part with?

[Sagaris]

Since you are running 195 65's right now, if you do decide to replace your tires with 60 series, it would make the 3.73 diff feel like a 3.85.

I went from 65's to 60's on my auto with 4.10, it makes the 4.10 feel like a 4.23. You can tell a difference, but it is very small. so going to a 3.73 you should certainly feel a difference, but you might not feel it much at all by running 60 series.

[QUKBMER]

Yes you will feel the difference driving on a 3:73 vs 4:10. I went from a 4:10 to 3:73 and in final gear was a 500 to 600 rpm difference around 75 mph was 2700 vs 3200/3300 rpm. It did accel slower from a stop and did not pull as well when going pass other people at any speed. I was trying to save gas but got disappointed by the performance and switched back to the 4:10. I am running 15x7 205/50-15 track tires and 16x7.5 205/50-16 street tires on my 91 325is semiautomatic. ;)

[tinkerputzer]

Thanks for sharing your first hand knowledge QUKBMER. Sounds like the gain in economy is not worth the drop in performance / drive-ability.