Diff Swap for better mileage?

[DaveCN]

I have a 1994 525iT with an Auto and the stock 4.10 diff.

I just got offered a job that will require me to commute about 65 miles each way to work.

One option I have is to sell the e34 and pick up a Civic or Metro. But I really would just like to wring out more mileage from the e34.

What I am thinking about is swapping in one of the following:

3.73
3.64
3.23
3.07

I am concerned that the 3.23 and the 3.07 may be just too long and leave me lugging the engine and not overall be the best for fuel efficiency.

Has anyone out there swapped in one of these diffs and seen an increase in mileage?

Thanks in advance for input.

[e34.535i.sport]

Hi there,

Haven't done a swap myself but I plan to in future... I'll be going higher (numerically) though for more acceleration. I saw this thread a while back when I was looking into it and thought it migth be useful-
http://www.bimmernut.com/forum/showthread.php?t=13240&highlight=diff+swap+mileage
The general consensus I think is that a diff swap is not the best way to increase mileage, as our friends at BMW have already found the best ratio for the fuel economy/performance balance.

From what I've read it's not wise to stray too far from what your car is blessed with originally either...

How about LPG?

[healtoeit]

Has anyone out there swapped in one of these diffs and seen an increase in mileage?


From what I have experienced (pops limited diff and gearing change for greater acceleration)
You do not want to go 30% out of the stock gearing...
it may seem like a small number but the effects are HUGE.
You have an automatic so I would not go lower then 3.71
I would suggest a 3.91.
This article might help
http://www.kormanfastbmw.com/tdiffgea.htm
There is a speed change change equation but i cannot remember it.
Your rpms will go lower if you go w/ a lower number diff... which I guess is what you want.

I would suggest a 6 speed conversion (I am looking into doing one, although it will be a long way away)... The guys w/ the 540i get about 25-27 on the highway (according to a tread last week).
I can only imagine what a 525i w/ a 6 speed would get on the highway!

[Ross]

Unless your driving is mostly highway cruising the gain will be small, the engine will require more throttle opening for comparable accelleration. My experience is that two and a half liters needs that 4.10 to lug it's fat touring ass around
You could try taller tires first.

[attack eagle]

x2 the auto doesn't have the balls or deep enough gearing to pull that thing around stop light to stoplight. plan a route that involve minimal stopping, and time the lights so you can get the next light without stopping.

[DaveCN]

OK - well - the 65 miles in the commute - 64 of it is highway driving. I could give a rats ass about stoplight to stoplight in my scenario.

I am going to plan on getting some 205-65-15s for the stock weaves and putting the 18" M-Parallels in the garage as a first step and see what improvement (if any) I see.

I have run the numbers in a spreadsheet that show the fairly dramatic changes in rpms at given speeds.

After I establish a baseline on the skinnier tires. I think I may start with a 3.73 and see how that goes. Given that open diffs are pretty cheap, It would not be a big deal to go taller if I chose after that.

Or, after the 3.73, look at going manual.


Thanks for your ideas, guys.

[healtoeit]

Or, after the 3.73, look at going manual.

The manual transmission for the m50 is geared differently then the automatic. The manual has a 3.23 diff in it...
running a 3.73 w/ manual trans would be great for autocross but very bad for gas consumption

[DaveCN]

Oh, no doubt. By "going manual" I meant the full monty - with an appropriate differential.

[healtoeit]

Oh, no doubt. By "going manual" I meant the full monty - with an appropriate differential.
well, why not do the manual swap now? I mean, it would be expensive, but it would be worth it!

[DaveCN]

Time constraints mainly.

I dont have time before this job starts to get it done, and unlike a diff swap, which I can manage in a couple of hours on the weekend without much fear of "Oh crap, I forgot ___________," or other such unplanned delays, the manual swap would be best put off until I have some vaction time accrued and/or alternate transport available just in case.

Given the way things sometimes go for me when working on cars, It'd be too risky for me to assume I can get the 5 speed swap done cleanly and without a hitch over a weekend.

In a perfect world, it'd have been done already.

[healtoeit]

This is true... and having a shop do it would just be too expensive.

[Paul in NZ]

gale has done several manual swaps...to get the best economy the motor needs to be sub 2000 rpm at 60 mph...the more economical versions of the M30 have higher gearing than the 3.64 in my 535

[attack eagle]

You could do the swap in a single weekend if you had all the parts lined up ahead of time... especially if you had a parts car. The manual swap and narrow lighter smaller size rims will make the biggest differences after a tune up.
You should pick up at least 1-2 mpg for the wheels, and probably 2-4mpg for the manual swap.
I did mine for about $1000 + $100-$150 in tools fluids etc.

90 minutes to swap the pedal box and do the electrical stuff inside the car, and about 5-6 hours to disassemble everything and about the same to reassemble, assuming garage floor and no help.
IF you have air, you cana knock out the swap in one day.

[DaveCN]

Garage floor? Heck no!

j/k

I have access to a lift and air tools.

I am going to plan a weekend to do it - it just won't be right away - have to source the parts still, and life is pretty crazy right now.

Thanks for the encouragement - I have looked over your writeup at bf.c more than a few times for info on the swap.

[bubba966]

If they made a 3.91 in that medium sized diff I'd go with that. I think the 3.73 might not get enough torque to the pavement for that to work well.

Funny I just picked up a 3.91 large case E34 LSD and am seriously wondering if it's going to be too deep.

Anyone want to trade a 3.73 large case LSD for my 3.91?

[632 Regal]

i went with the 3:64 and city mileage improved city but no gain/loss on the highway. I think 3000rpm is at 83MPH with the new setup. At first I freaked thinking more engine wear and bad mileage but I am comfortable with it now... 4K at 80 wouldnt scare me coz these engines are well balanced for higher rpms now low rpms like an idle is a different story.

[DaveCN]

another thought I am having - feedback appreciated:

If I do a 5 speed swap - I could use a Getrag 260 from an e30. It will bolt right up - a lot of the e30 guys use them when they put M50s in the e30.

AND - it has a true overdrive 5th gear.

Other than needing to figure out the driveshaft issue - should be as easy as using the ZF I would think.