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diesel swap MPG

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  • diesel swap MPG

    So I've been away for a good while and am looking at putting a GM 6.2 in my GW. I have tried to search and can't find organized info on this because of people leaving out tire sizes and gears. I would like to see what kind of mileage everyone is getting in all types of diesel swaps but please include any modification to the motors, tuning, drivetrain, gears and tire size. Thanks in advance.
    85 Grand Wagoneer
    MSD ignition/Edelbrock intake and carb

  • #2
    My 6x6 M715 with a 6BT gets about 13-14MPG.

    My 4BT-powered M715 gets about 18-20MPG.

    With a 6.2 and appropriate gearing, you should be near the 20MPG mark.
    1969 M715 6x6
    1963 J300 Swivel frame


    • #3
      What gears, tires, and transmissions do you run?
      85 Grand Wagoneer
      MSD ignition/Edelbrock intake and carb


      • #4
        If you're going for a dsl swap "just" for mpgs to save money That's generally not a viable economic plan. By the time you figure in total: buy-in eng costs, freshening up costs, accessories/mods etc costs, maint you're in it for thousands. You need a "sharp" pencil to figure how long/many miles you'll need to drive out the money spent just to break even or just gradually spend the money on gasoline. You heavy towing, doing mega long freeway trips? I haven't done the swap game...numbers never added up for me. I've had a couple 6.2's in their stock trucks and for work and mpg efficiency they like the 2000 RPM range. With a stock non-OD trans I've found 4.10 gears to work pretty well. Low to mid 20 mpgs avg all round use (rural, hwys etc but not towing or city traffic). With your crappy 85GW stock axle gearing you WILL need to re-gear your axles...$$$. Good luck with your "sharp pencil".
        "Don't mind me. I'm just here for the alibi"


        • #5
          To add to what Joe said - With diesel being much more expensive than regular gas, it will take a long, long time to break even.

          I built my trucks as a hobby. From a monetary point of view, I might as well have been throwing bundles of $100 bills into my fireplace
          1969 M715 6x6
          1963 J300 Swivel frame


          • #6
            My 1980 ScoutII (4BT-NV4500) gets low 20's in town and high 20's on the freeway. Towing a 3,500 RV it gets about 17-18 mpg.
            Tires are 33/10.50/15 BFG muds, 3.73 gears. It weights at least 4,500 lbs.
            I look at the swap as a hobby, don't what to know how much I've spent on it. But I've been driving it since 1999 so it's paid for it's self.
            Last edited by scout4bta; 04-24-2022, 05:50 AM.
            1989 GW/91.5 Dodge W250 CTD flatbed/ 06 Ford Crown Vic Police Intercepter/05 Jeep Liberty Diesel.


            • #7
              I have a 12V 6BT from 96 ram. Also used the OD 46RH. I've seen 20mpg going 55 with lockers, 4.10s and 37" hmmv tires. Once I go to 39.5 rockers that will change.
              '77 J truck.


              • #8
                First tank 20 mpg, 2nd tank 24, 3rd 21 mpg
                72 Wagoneer
                89 Isuzu 4BD1T
                4L80E (.75 OD)
                Dana 300
                D44 (4.30:1)
                31x10.5-15 BFG
                ~2100 RPM at 65 MPH

                Where I buy diesel, it's less than gasoline. It has been more, but over the years, it's close. So let's assume gas = diesel @ $3/gal. Also for easy numbers let's say a gas FSJ gets 10 mpg and a diesel gets 20 mpg.

                1 mile gas = $0.30
                1 mile diesel = $0.15

                I'll probably have $8,000 - 10,000 into my swap when it's done, (if it's ever done.)

                Swap cost/savings per mile=miles to break even on diesel
                10,000 / 0.15 = 66,666
                1972 Wagoneer > Isuzu 4BD1t > 4L80E > D300 > D44


                • #9
                  It's going to take a while to break even. If you already have a donor power train to swap then you're ahead of the game but still.. It's going to nickle and dime you to death.
                  I've got a 6bt/700r4. I can get mid 20's on the highway.

                  I could see it being worth it on these jeeps since they can get in the single digits. Just plan on keeping it a looong time or running wmo to recoup the costs a little faster.
                  89' GW 6BT Cummins


                  • #10
                    GM 6.5 turbo, 700R4, 3.31 gears, 235/75/15 tires. Consistent 21mpg in daily driving. Best tank was over 24 and I can consistently pull 23-24 on the highway. That was over 60k miles.
                    -87 Grand, 6.5L diesel, MHI TE06H turbo, Water/air intercooler, Art Carr 700R4, CS-130, hydroboost. 21/24mpg, dead 229 [Custom 242 on the way]
                    -99 XJ Limited.
                    -Jeepspeed 1717 -Built 4.0, FAST EFI, Rubicon Express, Bilstein, Rigid Industries, 4 Wheel Parts, G2 Axle, Currie Enterprises
                    -Member, FSJ Prissy Restoration Association