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Thread: Diff Swap for better mileage?

  1. #1
    Join Date
    Aug 2007
    Location
    KY
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    27

    Default Diff Swap for better mileage?

    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.

  2. #2
    Join Date
    Mar 2007
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    Liverpool UK
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    Default

    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/showt...f+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?
    1995 XJR: 4.0L S/charged straight 6 Auto

    What... It's not broken??? I can still fix it

  3. #3
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    Dec 2007
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    Northern(occupied) VA
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    Default

    Quote Originally Posted by DaveCN
    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!
    Conforti chip, K&N filter, Racing-dyn sways, EBC pad/rotor, SS brakelines, UUC DSSR, custom sound+alpine receiver, 17" PS2s, H&R/bilsports.
    "Speed is the product of doing everything right"

  4. #4
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    Apr 2006
    Location
    Chicago, Il. U.S.A.
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    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.
    "The gas pedal wouldn't go to the floor if it weren't meant to be there"

  5. #5
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    El Paso TX
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    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.

  6. #6
    Join Date
    Aug 2007
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    KY
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    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.

  7. #7
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    Quote Originally Posted by DaveCN
    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
    Conforti chip, K&N filter, Racing-dyn sways, EBC pad/rotor, SS brakelines, UUC DSSR, custom sound+alpine receiver, 17" PS2s, H&R/bilsports.
    "Speed is the product of doing everything right"

  8. #8
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    Oh, no doubt. By "going manual" I meant the full monty - with an appropriate differential.

  9. #9
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    Quote Originally Posted by DaveCN
    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!
    Conforti chip, K&N filter, Racing-dyn sways, EBC pad/rotor, SS brakelines, UUC DSSR, custom sound+alpine receiver, 17" PS2s, H&R/bilsports.
    "Speed is the product of doing everything right"

  10. #10
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    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.

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