Which is better 4.10 or 3.73 LSD?

[rcfanatic]

The graph says it's 3700..

[QUKBMER]

Anybody else have input on this 300 rpm difference? I don't see what all the hoopla is about

I have a 91 325is with an semiautomatic ;) and when i put a 3:73 in at 75mph i was at 2700 rpm's,put the 4:10 back in was turning 3100 rpm's. I have noticed a 500 rpm difference. My accel from a dig is alot better of course with the 4:10 but i didnt notice any change in mileage either. I drive 100 miles round trip 6 days a week and average 320 to 350 miles to one tank of gas. Now this may vary because most of you are running 5 speeds. I bought this car with an semiauto in it 9 years ago and mostly drive traffic thats why i left it in and havent changed it yet. I have thought of the ideal of leaving in the semiauto and booosting it.

[priapism]

is 80mph really 4000rpm? I don't think it is.. 75 is just over 3500rpm. But my tires are a little oversized.

Depends on the overall diameter of your wheels and tires as well. Running larger wheels will raise your overall gearing.

[nando]

it's not going to change it by 300rpm though, unless you have HUGE (or small) tires (which is what I think you meant, since wheel size has nothing to do with it).

[z31maniac]

Post #5 noobs, the difference is approx 12%.

So if you pull 3000 rpms at 60 with a 3.73, to find out your RPMS at 60 with a 4.10 take 3000 and multiply times 1.12 or ~3360.

(I made those up, just picked them to illustrate how to figure it out. It's not that hard guys.)

[rcfanatic]

Oh, post #5 is your post!.. So how did you calculate that 12% figure? oh wait.. ;)

[priapism]

it's not going to change it by 300rpm though, unless you have HUGE (or small) tires (which is what I think you meant, since wheel size has nothing to do with it).

It's the overall wheel & tire diameter.

[nando]

I know..

you'd need tires ~12% too small for it to be that big of a difference. Not to say lots of guys here don't run way undersized tires.

My tires are like 1% oversized. It's enough that the speedo actually reads dead on with my actual speed, instead of being over a few MPH.

[tjts1]

All the graphs are accurate with the specified tire size. I used a tire size calculator to generate these graphs when I was deciding what diff to use with my M42. I ended up with a 3.64. Best $50 I ever spent on that car.

[QUKBMER]

Depends on the overall diameter of your wheels and tires as well. Running larger wheels will raise your overall gearing.

205-50-16 and 205-50-15 did the same thing with both different wheels and tires.

[dashboardmonkey]

is 80mph really 4000rpm? I don't think it is.. 75 is just over 3500rpm. But my tires are a little oversized.

my tires may be slightly undersized

[nando]

more than slightly, that's like an 8% difference. :p

[dashboardmonkey]

They are pretty wore out, I have done ALOT of big smoky burnouts:pimp:

[801coppertop]

How do i tell what diff i have now?

[Atreus]

look at the tag on it. it might be dirty. if theres an S on it it'll make you feel good inside. lol.