Gear head question 3.73 0r 3.91

[Rave TT]

I've been thinking of doing a major upgrade that will change the performance and feel of my M5, I've been told that a new diff is the way to go. Should I get a 3.73 or 3.91 ratio. Thanks in advance for the advice.

[Rave TT]

Never mind about the 3.91, I found out it is meant for automatics, anyway should I go for the 3.73 diif?

[SilverStreak]

What's the OEM diff ratio in your car?

[Rave TT]

I think its 3.15

[SilverStreak]

Well, if you're gonna stick with OEM BMW parts, you could use a few LSD's. But if youre heart is set on 3.91's, get a Quaife LSD, and you can choose whatever gears you want (pratically) and it;s one of the best LSD's out there.

[Kevlar]

Check what DINAN uses for their differential upgrade. I think they use a 3.46 LSD. I can't remember as I'm not really up to speed on M5 stuff.

[Rave TT]

Yes, Dinan does use a 3.45 diff, my thinking was if 3.45 is good 3.73 would be better. 3.91 is meant for automatic trans. so that is out for me.

[Kevlar]

yeah, but remember, if you start going higher IE 3.73 or 3.91... you're going to be changing gears alot more. I DINAN uses the 3.46 diff because it put 60mph at the top of second gear.

I'm not sure what 6th gear is like on the M5, so you're RPMs may be much higher while cruising too.

[SilverStreak]

Yeah, and you can get whatever gears you want, you can get a 4.44 if you want, if you don't mind being at 4000 rpm at 55 mph in 6th gear...

If you want, play with this formula to figure out your speeds in each gear at whatever rpm, etc...

(RPM x Tire Diameter*) / Rear Differential Ratio x Transmission Gear Ratio x 336 = MPH

*Tire Diameter- for instance with 245/40-17:

[(245 x .40 x 2) / 25.4] + 17 = 24.72"

[Drea///M3]

Originally posted by Kevlar
it put 60mph at the top of second gear.



Juast wonderin' if that's "a rule of thumb?" To put 60mph at the top of 2nd gear? I'm asking 'cause I too am looking into gears but for my '97M3/4 Slushbox...I'm looking at 3.73s which looks safe and good, but others have been running 3.91s and says it "rocks." Most 3.91s are on '95 autos though, I don't know if their trans gears are different than my year....Anyadvise? I'm looking for the best improvement in performance but keep street/highway driveability....Thanks!!!!

[SDbboy]

Originally posted by SilverStreak
Yeah, and you can get whatever gears you want, you can get a 4.44 if you want, if you don't mind being at 4000 rpm at 55 mph in 6th gear...

If you want, play with this formula to figure out your speeds in each gear at whatever rpm, etc...

(RPM x Tire Diameter*) / Rear Differential Ratio x Transmission Gear Ratio x 336 = MPH

*Tire Diameter- for instance with 245/40-17:

[(245 x .40 x 2) / 25.4] + 17 = 24.72"


Dave what is the 366 constant called?

Is Rear Differential Ratio = to Final Drive

Also how do I find out Transmission Gear Ratio for the E46 M3?

Thanks, Brian

[ihooklow]

Ok, you guys are messin' me up. Is the constant 336 or 366?

Thanks.

[SDbboy]

My mistake, it should be 336.

Brian

[onebadm5]

i considered switching to the dinan 3.45, but i didnt wanna give up the top end.

[SilverStreak]

Yes, 336. I forget what it's called! :D I just know the formula works. :)

Look at BMW's main website, or a brochure might have it, or ask some of our members in the E46 M3 Forum about the ratios.