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    #1

    Differential Swap

    Currently I have a 1991 318i with a 4.10. I'm not sure if it's open or LSD. I'm getting 25mpg on average and it's not great. I had a 318is that would easily get over 30mpg but that car was a lot more modified. This car is bone stock.

    I'm not crazy about speed right now. Just looking to get better fuel economy so I can justify to my wife that this 23 year old car was a "good" purchase.

    What is the lowest ratio diff you would swap into this car? Is a 2.73 LSD going to make it sluggish? I'm thinking low like a 3.xx LSD.
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    #2
    This has been covered many times on M42Club. The consensus seems to be that you should get a 3.73 as the best compromise, or a 3.46 if you are crazy about hypermiling. Anything numerically lower is going to be awful.

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      #3
      Thanks! I don't frequent M42 club. As I've glanced over there it seems to be a lot of E36 cars so I thought I'd ask here.

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        #4
        If you do run a 3.xx differential you should probably chip the car to make up for some of the losses. Thus far with the two cars I have with chips (MarkD, and an old Turner?) I have not noticed a real change in mileage.

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          #5
          That's my plan. If I can find a 3.64 or 3.73 LSD I'm going to grab it and then chip the car. Thanks so much for your info!

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            #6
            How many miles are you driving it on the freeway monthly? this probably won't save you much money in the long run.

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              #7
              I was thinking 3.64 as well...

              t
              now, sometimes I just mess with people. It's more entertaining that way. george graves

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                #8
                Originally posted by roguetoaster View Post
                This has been covered many times on M42Club. The consensus seems to be that you should get a 3.73 as the best compromise, or a 3.46 if you are crazy about hypermiling. Anything numerically lower is going to be awful.
                Not true.


                I've run 2.93lsd in two stock M42 cars. That ratio is fine for freeway driving. never had issues getting around town either. The cars also got great mileage.

                Turbo M42 Build Thread :Here
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                I like the tuna here.
                Originally posted by lambo
                Buttchug. The official poster child of r3v.

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                  #9
                  Originally posted by Dj Buttchug View Post
                  Not true.


                  I've run 2.93lsd in two stock M42 cars. That ratio is fine for freeway driving. never had issues getting around town either. The cars also got great mileage.
                  Well, I suppose it's a matter of personal preference. When I put a 3.15 in my stock+chip 318 it really neutered the character of the car. Yes, it's good for cruising on the highway, but to make a pass you really have to downshift one or two gears. Maybe not a problem in some areas, but there is a safety aspect to this.

                  Now that I own a car with an M40 it felt very similar in character with a 3.15 in an M42.

                  There is also the issue of driveline wear with a ratio that is too far off of the original as you really need to wind the car up to get moving. I felt this was a problem with the 3.15 too. OTOH, with the 4.1 you are running the car at 3.5k+ all the time on the highway.

                  To each their own, but it seemed that the consensus on M42club was that 3.73 was a good step down without too many downsides.

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                    #10
                    Originally posted by BraveUlysses View Post
                    How many miles are you driving it on the freeway monthly? this probably won't save you much money in the long run.
                    My drive to and from work is all freeway. So roughly 700 miles of freeway monthly.

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                      #11
                      Just to give you an idea of how little this will change things:



                      I mean if your 4.10 LSD is dying then yeah it might be a no brainer to swap it but you will take a while to recoup your costs.

                      2mpg difference over 700 miles only saves you 5 bucks a month.

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                        #12
                        I have a 3.91 vlsd on mine and love it. Nice for freeway driving and comfortable shifting in daily situations

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                          #13
                          Originally posted by LinderM3 View Post
                          Currently I have a 1991 318i with a 4.10. I'm not sure if it's open or LSD. I'm getting 25mpg on average and it's not great. I had a 318is that would easily get over 30mpg but that car was a lot more modified. This car is bone stock.

                          I'm not crazy about speed right now. Just looking to get better fuel economy so I can justify to my wife that this 23 year old car was a "good" purchase.

                          What is the lowest ratio diff you would swap into this car? Is a 2.73 LSD going to make it sluggish? I'm thinking low like a 3.xx LSD.
                          i am using 3.91 LSD on my e30 M42 and i get roughly 10 km a liter. on highways i get 10.5 to 11. car runs on standard engine no mods. engine comes from e36 with DISA

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