89 325i

[new2bmw]

I have an 5 speed 89 325i and may need a new differential. Was just wondering what would be the ideal differential to use? i know LSD is the best but if I cant afford one what would be the next best option and ratio. I drive mostly in town but do make frequent road trips, if that makes any difference in my prospects.
Larry

[TopOfTheLine89]

If you want epic highway cruising comfort,MPG,and to hit 100mph in the top of 3rd gear get a 2.93 LSD. If you want the car to remain zippy just get another 3.73,If you want 1st gear to be pointless and the car to be stupid get a 4.10. Any other options would require you to change flanges.

[new2bmw]

4.10 ?

So a 3.73 is probably best suited as it is stock, what is the issue with the 4.10? Sorry for my ignorance, never had to deal with a diff before. You say it will make the car stupid. Can you humor me and explain a bit more?
thanks
Larry

[Hallen]

With a 4.10 you are shifting out of first to second at about 30mph at red line, you shift from second to third at about 52mph. You cruise in 5th gear down the freeway at 70mph at 3350RPM.

With a 3.73 you are shifting out of first to second at about 33mph at red line, you shift from second to third at about 57mph. You cruise in 5th gear down the freeway at 70mph at 3050RPM.

(Both above assuming stock wheels and tires)

So you can see the difference in the numbers isn't huge. But you will have to shift sooner and more frequently with a 4.10 diff. It will be noticeable. A 4.10 will get you a bit more torque and it is quicker for some applications like track cars and racers. But even that depends on the track.

The 3.73 is going to be a bit more smooth and you won't tach out as often in normal driving. It will be a tad quieter at higher speeds and you'll get better mileage.

Those are the trade offs.

Final gear ratio is simply your current gear ratio for the gear you are in multiplied by the final drive (diff) ratio. The bigger the diff ratio, the bigger that final number is. Take that ratio and divide it into your current rpm's and you get the rpm's at the rear wheels. Multiply that number by the total rolling circumference of your rear wheels, and you get how fast you are going. It's a nice way to check your speedo :)

[Adrian_Visser]

325i diff ratio is 3.25:1 lsd is an option
325is is 3.73:1 lsd
med case 4.10:1 lsd diff's are hard to find only used on auto with the lsd option or m3's afaik.

[Julien]

325i diff ratio is 3.25:1

no

[xLibelle]

why do i see guys talking about 4.11 diffs?? where is the confusion coming from?