How to determine rear diff ratio?

[Chris E30 Cab]

Can anyone explain how you go about determining a suitable rear diff ratio?

The 3.73LSD I'm using with my 3.0L stroker has too short of a 1st gear. Currently I cannot even get to 100kmh (60mph) in 2nd gear.....I have to change to 3rd. There is plenty of torque with this engine so I'd like to get a more suitable rear ratio, any thoughts?

I'm thinking a 3.64 would be ideal but having problems finding one locally.....whaddya think about a 3.45 LSD?

Does anyone know what rear ratio the E28 535i have?

And finally, how does the gearing of the transmission affect the diff ratio?

BTW, I do not have the Getrag 260 (325i gearbox), but have a Getrag 240 (E30 320i gearbox).

Thanks!

EDIT: Ted, how's your 1st gear? Can you get to 60mph in 2nd gear? I assume you have the Getrag 260, correct? Reason I ask is that I am aware of the different gearing in the first 3 gears between a Getrag 260 & 240.........I just have no clue how all this works :?

[SlammedE30]

I bet a 3.64 or 3.46 would be fine. 1st is awfully short, and I can't hit 60 in 2nd either, but I like the 3.73 for track use. I have a Getrag 260.

I seem to recall a E28 or E24 (6er, if it's wrong I blame the 1AM brainfart) that used a 3.46, but I'm not sure which one.

The final drive has nothing to do with the actual gears, but it changes the amount of times the tire revolves per revolution of the driveshaft or something like that. So a 3.73 would turn the tires one time every 3.73 revolutions of the driveshaft. I think that's right. Hope I got most of what you're looking for.
-Ted

[Brew]

I've got the getrag 260 in my 325is and I can just barely hit 60 in second gear with a 6900rpm redline and 3.73LSD final drive, if that helps at all.

[Deutschmarque]

this may help

http://www.unixnerd.demon.co.uk/revs.html

-Nick

[E30godz]

i think its more like 58 mph, not 60...even at 6900 rpm

[Jonathan M3]

I know the 3.46 was in the automatic e28 528e, lsd was optional. Not sure what other, if any, bimmers had this ratio.

What are the gear ratios in your 320i tranny, Chris?

[bimmerphile]

Jon-

E46 323/C/i/c/, 325/C/i/c/, 330/C/i/c automatics have a 3.46 rear end.

As do the 330/c/i ZHP's and the M3 CSL

[Chris E30 Cab]

Thanks fellas, perhaps I'll give it a go.

Jonathan, I'm unsure of the ratio's, but I found them on the web once and need to find the link in my favorites list. Although, I'm quite sure It is the same gearbox that is found in the M10 powered 318i.

[E30/M30]

Just copied this,HTH

525i M20 89-91 5 speed 3.73
525i M20 89-91 Automatic 4.10
525i M50 All 3.23
535i 5 speed 3.46
535i ->6/90 Automatic 3.91
535i 7/90-> Automatic 4.27
530i 5 speed (M60) 3.07
530i Automatic (M60) 3.15
530i 5 speed (M30) 3.64
540i ->3/04 2.93
540i 4/94-> 2.81
M5 3.6 ->7/91 3.91
M5 3.6 7/91-> 3.73

[KenC]

Just copied this,HTH

525i M20 89-91 5 speed 3.73
525i M20 89-91 Automatic 4.10
525i M50 All 3.23
535i 5 speed 3.46
535i ->6/90 Automatic 3.91
535i 7/90-> Automatic 4.27
530i 5 speed (M60) 3.07
530i Automatic (M60) 3.15
530i 5 speed (M30) 3.64
540i ->3/04 2.93
540i 4/94-> 2.81
M5 3.6 ->7/91 3.91
M5 3.6 7/91-> 3.73

Where did you get those? I'd like to see what my '84 318 is (5-speed).

[E30/M30]

Sorry,it only list's E34's

[Chris E30 Cab]

Found the gearbox ratio's for my Getrag 240:
Ratios:1 (1/2/3/4/5/R): 3.72 / 2.02 / 1.32 / 1.00 / 0.81 / 3.45
Borrowed from Fred's great site below:

http://www.strictlyeta.net/technical/specifications2.html

Will put the details into the calculator, thanks for the link, but it ignores the power & torque of the engine. Metric Mechanic offers a formula for determining the ideal ratio (link is below) but it ignores the gearbox ratios:

http://www.metricmechanic.com/pg77.htm

Hence my confusion????

As I mentioned before this is the same gearbox that is found in the early 318i (M10 powered)