What MPG are you 340 guys getting?

Small Block Mopar Engine

  1. RustyRatRod

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

    Messages:
    79,449
    Likes Received:
    57884
    Joined:
    Jun 7, 2010
    Location:
    Georgia
    Local Time:
    8:44 PM
    Dang skippy.
     
  2. AJ/FormS

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

    Messages:
    21,770
    Likes Received:
    9020
    Joined:
    Jan 19, 2014
    Location:
    South-Central Manitoba,Canada, 900ftelevation
    Local Time:
    7:44 PM
    Some,
    to a lot,
    of fuel mileage gains
    can be found in the cruise timing. Sometimes (depending on the cam) it is possible to run the engine at too low an rpm, and to not be able to give her the ignition timing she desperately wants.
    For instance, if you gear your car for 65=2000, and the engine wants 52* of timing...... how are you gonna meet that need? With factory parts, it is nearly impossible. What if your cam is big enough to still be in reversion at 2000? What if, because of that reversion, your Effective Dynamic Compression ratio is just 4 or 5/1?
    It's all in the combo, and especially in the ignition timing.
    A manual trans, traditionally, has been cited to make about 10% better fuel economy, cruising at the same rpm. Part of that is in the TC slip; and that is why the factories have gone to loc-up convertors. And another part is inside the autos themselves. And a third part is in the transmission ratios.

    Another major factor in fuel economy is your running cylinder pressure, which is governed by your Dcr (Dynamic Compression Ratio), the Ica (intake closing angle), and the throttle opening. This runs in conjunction with your carb, and carb size. If you gear your cruise rpm too low, and your throttle opening becomes too large, then she will get up on the mainjets. But now, the airflow thru the venturies will be quite low, because the power-requirement is so low. And it won't take much error in MJ selection to totally negate, or worse, all the work you put into the low-rpm running.
    I used to run hi-compression 340s for years, beginning in 1970, in hi-school. 340s were never known for fuel economy except to say it was lousy compared to any other Mopar offering. And part of the problem, perhaps most of it, was the factory 268/276/114 cam.
    That's part of the reason, as a streeter, I switched to 360s years ago. And the biggest reason I installed alloy heads on mine, was to get the super-high cylinder pressure..
     
    Last edited: Aug 27, 2021
  3. Gator

    Gator Mopar or No Car

    Messages:
    229
    Likes Received:
    23
    Joined:
    Jul 1, 2007
    Location:
    Meadville, Pa
    Local Time:
    8:44 PM
    10 if I dont hammer it and if I do I get about 8 in my Hi Po 340...
     
  4. 66fs

    66fs FABO Gold Member FABO Gold Member

    Messages:
    6,148
    Likes Received:
    2658
    Joined:
    Sep 9, 2009
    Location:
    Greenwood, South Carolina
    Local Time:
    8:44 PM
    Back in the day, 71 340 Duster, 4 speed, 2.76 gears, 20-21 mpg highway. I did have other gears that did not do as well.
     
  5. LO23M8B

    LO23M8B Well-Known Member

    Messages:
    1,435
    Likes Received:
    721
    Joined:
    Jan 30, 2019
    Location:
    Michigan
    Local Time:
    5:44 PM
    Back in the day 340 4 speed all stock except carb 3.23's 20 all the time average. Another car 340 4 speed modified (small) 4.10's in the city about 16-18.
     
  6. Dartsun

    Dartsun Mopar Dude

    Messages:
    2,141
    Likes Received:
    1927
    Joined:
    Feb 12, 2012
    Location:
    Springfield Oregon
    Local Time:
    5:44 PM
    My 340 in my 67 Belvedere 4 spd 3:23. Got around 10mpg combined. Mild driving edelbrock 600cfm.
     
  7. Nat

    Nat Well-Known Member

    Messages:
    754
    Likes Received:
    416
    Joined:
    Jun 11, 2019
    Location:
    Canada
    Local Time:
    1:44 AM
    my 340 mild build got 22+ mpg with 3:55 and a gv overdrive
     
  8. rumblefish360

    rumblefish360 I have escaped the evil Empire State! FABO Gold Member

    Messages:
    41,701
    Likes Received:
    13950
    Joined:
    Jun 21, 2005
    Location:
    Florida
    Local Time:
    8:44 PM
    Ahhhhhh, there’s that GV/OD unit again!
    If these things are in your budget, there really cool.
    Expensive stuff.
     
    • Like Like x 2
    • Yote

      Yote Well-Known Member

      Messages:
      1,178
      Likes Received:
      486
      Joined:
      Aug 30, 2015
      Location:
      far eastern Colorado
      Local Time:
      6:44 PM
      340, 4 speed, 3:55 rear - my last 90 mile steady drive got 17 mpg
       
    • 512Stroker

      512Stroker We are all here because we are not all there.

      Messages:
      3,129
      Likes Received:
      2474
      Joined:
      May 30, 2016
      Location:
      Freedom, MO
      Local Time:
      7:44 PM
      AHHH! My 67 LA 340 Barracuda gets double digit fuel mileage.
      Like 9.2-9.8 mpg on the highway going down hill with a strong tail wind.
       
    • 72Dart6pack

      72Dart6pack Harder Better Faster Stronger.

      Messages:
      4,758
      Likes Received:
      5291
      Joined:
      Feb 1, 2016
      Location:
      Roseville Michigan
      Local Time:
      8:44 PM
      I'm in the 7 mpg range. And happy as hell.
       
      • Agree Agree x 1
      • dadsbee

        dadsbee FABO Gold Member FABO Gold Member

        Messages:
        1,230
        Likes Received:
        2723
        Joined:
        Nov 25, 2020
        Location:
        Penetang, Ontario
        Local Time:
        8:44 PM
        Last Dart cruise I put in 7.5 gallons, drove 70 miles and came back with the gauge back in the same place. 9.3MPG and I was being pretty gentle with it.. lol.
         
      • K.O. SWINGER

        K.O. SWINGER Meeting in the alley since 1976

        Messages:
        2,335
        Likes Received:
        2000
        Joined:
        Nov 18, 2010
        Location:
        oregon
        Local Time:
        7:44 PM
        Anywhere from 4 to 16 mpg. 391 rear end and 240 at 50 camshaft oh yeah I run a over jetted six pack :lol:
         
      1. This site uses cookies to help personalize content, tailor your experience and to keep you logged in if you register.
        By continuing to use this site, you are consenting to our use of cookies.