Big Thread on Diff Gearing vs. RPM's

[Moron95M3]

A lot has been talked about regarding diff ratios and cruising RPM. Here is a lot of info that I threw together.

If you want the actual spreadsheet, let me know, I'll send it to you

Also, this was based on a 235/40/17 tire (stock for 95, and what I happen to be running )

Trans gearing was based on: (should be correct for US 5sp M's - autos/6spds are all different, even the euro 5sp I believe is slightly different)

1: 4.2
2: 2.49
3: 1.66
4: 1.24
5: 1

So here's the data:

2.93 -



3.15


3.23


3.38


3.46


3.64


3.73


3.91





And now here are the gear compared to eachother:

1st:


2nd


3rd


4th


5th


And because for a street/track car, we're worried about cruise speeds in 5th...

[TheHashmaster]

nice write-up

[Brent 930]

Nice charts.

I think it proves either 3.38 or 3.46 is the best all around diff ratio for street for more pep.

[zipaquira]

Yes, nice report, thanks.
I got the 3.46 and the rpms correspond to what was shown.

[shaeff]

I <3 my 3.38. Thanks for sharing, Moron.

(I feel so mean saying that!)

[WRXEATR]

Your 3,91 graph is off by about ~5 mph or so......But very nice info nonetheless.

[MIMI1]

4.10 or adios

[MauiM3Mania]

4.10 or adios

Why hold back? 4:45 all the way a la ABL.

I compromised with the sedate 3.91.

[JETninja]

Happy with my 3.23....cruises nice at 85mph every morning on my 35m each way commute, and since I'm a tall skinny Mofo at 142lbs...it moves darn well unlike most you fat bastards! J/K

Nice work Dude!

[MIMI1]

Why hold back? 4:45 all the way a la ABL.

I compromised with the sedate 3.91.

I just picked up a 3.91 too

[ninjaman111]

Can anyone with a six speed (euro, e46 or z3) tell me what there highway cruising speed is with either a 3.46 or 3.64?

Thanks

[Moron95M3]

Can anyone with a six speed (euro, e46 or z3) tell me what there highway cruising speed is with either a 3.46 or 3.64?

Thanks

simple.

the 6th gear ratio is .83. so take any 5th gear number above and multiply it by .83. (5th gear ratio is 1, so it's simple)

in the ball park of 2800rpm (mathematically)


and thanks guys for the comments!!

If numbers are off, that's due to differences in tire size (I know a lot of ppl are on 245s or wider, or on 18's etc).

[racer m]

Your 3,91 graph is off by about ~5 mph or so......But very nice info nonetheless.

auto
is 3500 rpm, is near 90 mph

Why hold back? 4:45 all the way a la ABL.

I compromised with the sedate 3.91.

I like this

Happy with my 3.23....cruises nice at 85mph every morning on my 35m each way commute, and since I'm a tall skinny Mofo at 142lbs...it moves darn well unlike most you fat bastards! J/K

Nice work Dude!

I am at 190 LBS. but I M 6'2".

I just picked up a 3.91 too

CONGRATS!

[95s52]

For more information on the rpms with different rear ends AND custom rim and tire set ups, download the Excel spreadsheet offered at diffsonline http://diffsonline.com/bmw-different...formation.html

[Volf]

For more information on the rpms with different rear ends AND custom rim and tire set ups, download the Excel spreadsheet offered at diffsonline http://diffsonline.com/techinfo/index.shtml

I was going to post this too. Its the same information only not graphed out.

[JETninja]

You peeps do know that as speeds go up, so does the speedometer error. At an indicated 90mph, your probably going closer to an actual 85mph....

[MauiM3Mania]

You peeps do know that as speeds go up, so does the speedometer error. At an indicated 90mph, your probably going closer to an actual 85mph....

Road speed is determined by rear axle speed (rememnber that sensor back there?), so diff ratio has no effect.

[JETninja]

Road speed is determined by rear axle speed (rememnber that sensor back there?), so diff ratio has no effect.

I didn't say the diff ratio had any effect, I'm just saying that speeds'o all have some error, and it grows with speed. And I've only seen them go on the plus side. Have driven by many a Speed Warning sensor (got one at work next to the airport in fact that I pass every day) where I get X band pop-up of my actual speed...within its calibrated zone also. Ha Ha...but if I'm doing an indicated 40mph, they always saw I'm doing 38mph...get a little different reading with each of my 4 vehicles.

Seriously, do you know how many diffs I've opened up! I know these things way too much!

[MauiM3Mania]

Sorry John, my mistake!

[JETninja]

No biggie Dude! BTW, how are the Islands these days? Been too long since I've been there!

[racer m]

You peeps do know that as speeds go up, so does the speedometer error. At an indicated 90mph, your probably going closer to an actual 85mph....

My speedo is +2mph at 80mph(gps said i was going 78 mph).

[JETninja]

Thats pretty good....but GPS is only as good as how many Sats it can lock into. Speed is not the greatest concern of the software, just a cool bonus.

[Brent 930]

I also see -2 MPH from speedo to GPS.

[ganzibye]

good info.thanks.i think i have got 3.23 ratio as stock oem,right ?

[MauiM3Mania]

No biggie Dude! BTW, how are the Islands these days? Been too long since I've been there!

Awesome. 85F with trade winds during the day, 70F and calm nights. Great time to visit as the economy has hit to a degree.



With a maximum speed on Maui of 55, the 3.91/six speed combo is fine and would be OK with a five speed. (staying on topic)