Best possible mileage?

Slant 6 Engines

  1. 89on35s

    89on35s Inferno Red Duster FABO Gold Member

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    I am thinking about buying an 84 Dodge Short bed 2wd Dodge pick up with a 225, 4spd O.D. standard trans and whatever rear gear ratio that it would have come with. I am willing to optimize whatever I need in an effort to maximize highway mileage.
    What type of mileage is this thing capable of? What could be done to make it the best it can be? I am new to slants, as I only ever had an 81 Diplomat with a 225 many years ago.

    Cley
     
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    • 89on35s

      89on35s Inferno Red Duster FABO Gold Member

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      Oh and the 225 is a fresh rebuild by a the owner who is a machinist and very good mechanic.

      Cley
       
    • dirty white boy

      dirty white boy 50 yr old Juvenal delinquent

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      i got an 80 model short step bed D150 stock /6 4 speed od truck, my daily driver, gets 18 mpg around town and bout the same at 65+ on hwy, but bout 22 mpg cruising the back roads at 45 to 55!! and that not even trying!! no idea how its timed, ant ever run over head, ant ever pulled the plugs, thing just keeps going, out run the energizer bunny!!
       
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      • el5dart

        el5dart Well-Known Member

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        i used to get 22mpg out of my 69 vert barracuda 318 2bbl 4speed back in the 80s. my current dart is only getting 10 city 13 hwy right now. pretty bad mpg comparatively. i will be working on mpg this summer for sure.
         
      • my68barracuda

        my68barracuda Well-Known Member

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        My 83 slant powered short bed D150, a833 OD transmission with 3:73 gears will get 15 mpg in every day city stop and go driving. On the highway, keep the speed at 65 or below it will get in the low 20’s mpg. Push it it to 75 and above and the mpg will drop to the 17’s. I could probably get better milage numbers, as I don’t drive this truck ez.
        The 80’s D150’s are great trucks, but they are also heavy and as aerodynamic as a brick.
        About anything on the road today will get substantially better mileage. To get the best mileage keep the tires properly inflated and drive it with a light foot on the throttle.
         
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        • el5dart

          el5dart Well-Known Member

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          i had the performance vacuum gauge in the 69 i used to get a kick out of keeping the vacuum as high as i could. for mileage
           
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          • 318willrun

            318willrun Utube channel 318willrun

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            I had a 225/4-speed with 3.91 gears in a '74 D100. I got 24-26 mpg. Unbelievable, but we checked it several times.
             
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            • el5dart

              el5dart Well-Known Member

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              my 2000 lwb conversion van with efi 5.2 magnum is getting 16mpg and weighs probably 2k lbs more than the 71 dart so surely the dart can improve
               
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              • pishta

                pishta I know I'm right....

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                Dart Lite/Feather Duster claimed mid 30's with a slant: lean carb and a reworked distributor.
                 
              • Slantsix64

                Slantsix64 Well-Known Member

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                Take all the smog stuff off, lean burn if it’s got it. Recurve the distributor and get as much vacuum advance you can get Before she pings. If you want to do deeper up the compression by milling the cylinder head down etc. I get 22 mpg with my 64 dart 4 speed headers four barrel with 3.55 gears!
                 
              • Kendog 170

                Kendog 170 Let the boy go !

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                I had a 83 Dodge pick up slant six OD 4 speed. Never really checked the mileage but did my ritual comute every week and only had to fill up once a week with Premium. On regular I would have to gas up sooner so for whatever reason I got more miles per tank with super.
                 
              • RustyRatRod

                RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                I believe 3.55 is the "standard" gear, but there were others. In general, about as good as they will get is "around" 20 give or take. That's pretty good for a truck. Honestly, a 318 would probably get better with more power, but the cool points go to the slant six.
                 
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                • BillGrissom

                  BillGrissom Well-Known Member

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                  I kept careful notes on my 1969 Dart 225 which I owned 1976 until 1994 (stolen). I got 22 mpg highway, but the speed limit was 55 mph back then. Around 1988 I changed to electronic ignition (Crane Cams XR700), but noticed no difference from the points. The Holley 1920 carb was always finicky, running too lean at idle. Might have done better with larger OD tires.
                   
                • rumblefish360

                  rumblefish360 So close, yet so far away FABO Gold Member

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                  I wonder what happened to all the”30 - 35+mpg guys” I used to call BS on.
                   
                • RustyRatRod

                  RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                  They were lyin mofos.
                   
                • rumblefish360

                  rumblefish360 So close, yet so far away FABO Gold Member

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                  LMAO!

                  How about that guy, he loved the “Plastic Fantastic” TQ carb, dang, his screen name escapes me at the moment.

                  Any ways.... claimed 42 MPG’s with a TQ. 53+ on his slant 6. No overdrive.... Swears Chrysler had literature on the subject out circulating in print.
                   
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                  • Brooks James

                    Brooks James Dustoff

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                    I drove my /6 (stock) from Nevada to fl and averaged 21
                     
                  • TexasDart

                    TexasDart Well-Known Member

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                    My buddies 340 six pac got better gas mileage than my /6 I had in my 65 Dart. I would get about 18-20 on the Highway.
                     
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                    • Brooks James

                      Brooks James Dustoff

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                      My old 93 Dakota w od only got 18
                       
                    • RustyRatRod

                      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                      Those things are heavy little pigs, too.
                       
                    • Brooks James

                      Brooks James Dustoff

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                      True I put 250k on it without a major repair
                       
                    • AJ/FormS

                      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                      I'm here,lol.
                       
                    • RustyRatRod

                      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                      I knew you were. LMAO
                       
                    • AJ/FormS

                      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                      Martinsville..... yur only 8.5 hours from me.
                      As to the engine; Do this:
                      Get the thing tuned up and running right. Put a 195 stat in it.If it overheats fix it, but make it work with the 195 stat. If it has flat tappets, set them at that temp to .013/.022.
                      Next; you have to make sure the engine gets the maximum amount of CRUISE-Timing possible, that it tells you it WANTS, not what you might think it wants.
                      So, Make sure the V-can is working.
                      IMO, you can leave all the pollution junk on there for now. If it has a lean-burn, make sure the Vacuum pod is functioning. Make sure the Vcan works in Neutral.
                      Ok next you will need a timing lite and a tachometer.
                      I'm guessing your combo will cruise at or near 65= 2200.
                      So at running temperature; in neutral, put it up on the fast idle cam at around 2200 rpm. Then just pull in timing until it stops picking up rpm; don't even put the lite on yet. Then reduce the rpm back to 2200. Next, RETARD the timing some until the rpm peaks again. But if the engine loses rpm, then advance it again, until it stops picking up rpm. And repeat until you find the max timing that makes 2200 rpm.
                      When the rpm peaks at around 2200 rpm NOW read the timing.
                      Then retard it three degrees and write the number down. Then return the engine to idle.
                      Now, if you run outta slot on the distributor before the rpm peaks; modify the slots! for more advance. How will you know ? IDK, I'll guess your slanty will TOLERATE about 42* or more at 2200 in Neutral.
                      What you have just done, is obtain the maximum advance that your particular combo, WANTS or TOLERATES, at 2200rpm.
                      Now you just gotta figure out how to make it happen. ( A new post or thread)

                      As to the chassis:
                      It is a brick!
                      I got an 84 with a 318, and the faster I drive it the more gas it sucks....... cuz it has to punch a pretty big hole in the air. So you can't do much about that. But there are some things you can do.
                      > Hard tire pressures as mentioned
                      > alignment
                      > make sure brakes are not dragging and wheelbearing grease ain't turned to soap.
                      > install new U-joints and repack them before installing, with Moly grease
                      Install 75/90Gear oil in the rear and STRAIGHT Dextron-II in the trans.
                      > some say leave the tailgate at home.
                      > turn the A/C off, keep the windows rolled up. If you have to make a choice, use the A/C
                      > that's about all I can think of. There are more things you can do, but none are cheap. These will get to to 80 or 90%

                      Once you have this baselined, then you can map out your timing curves, and see if there's a lil more left in the tune.
                      Or I'll sell you my 84 and you can steal parts,lol.
                       
                      Last edited: Feb 16, 2021
                    • TexasDart

                      TexasDart Well-Known Member

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                      I run a tonneau on my F150 and keep my tailgate. It’s very economical. I don’t go anywhere. Poor truck just kinda sets there. It was a little whiter today.

                      4EC57FE0-EB34-4DC6-BF6C-9B4242264AD2.jpeg
                       
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