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Fuel economy woes.

Discussion in '1982-Present GM Diesel' started by colbystephens, Nov 25, 2006.

  1. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    just drove to and from seattle and got only 13 mpg! :eek1: i really don't think it should be that low. i've got the 6.2/465/410/35's. at 60 mph, i'm turning 2361 - which is kinda high, but not too bad. i still think i should be getting better. what can affect fuel economy? how much will it help fuel economy to get a k/n style filter? currently my intake is not routed anywhere - it just takes in air from the engine compartment. How much would it help to direct air ducting to the front of the truck at the grille? when i rebuilt my engine 6K ago, i installed the injection pump w/ out replacing the copper washers. could this hurt fuel economy? i run my tires at 22-23 psi, and i'm not too interested in increasing the tire pressure. i think the gear oil in the tranny is super old - if i change that will it help? i need to do it any way, i suppose. i also think my timing is slightly retarded - could this affect my fuel economy? i have plenty of power (infact, i'm real impressed by it) - so i'm sort of dismissing any real tuning problems, but maybe i shouldn't be. Just seems that if something were wrong enough with my tuning to affect fuel mileage that much, i should be feeling it as a lack of power. thanks for any ideas!
     
    Last edited: Nov 25, 2006
  2. SLOK5

    SLOK5 1/2 ton status

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    OVER DRIVE AND AIR UP YOUR TIRES .I turn 1350 at 60 mph 700r4 33" 373 ...
     
  3. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    neither of those are gonna happen. i like to keep the tread on my tires, and i'll never run anything but a manual. won't be getting an NV4500 any time soon either.

    fwiw, tRustyk5 had this set up but w/ 4.56's and 40" tires and he still was getting 16 mpg.
     
  4. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    Yeah, I'm betting if those R's where 300 lower you'd see a substantial gain.. Time for some 37's! :wink1:

    And yeah, start running those tires at 32 to 35 and that will help a bunch... I always like to 32 to 35 on the street...
     
  5. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    What are you talking about? You'll get better tread life at 32 to 35 than low 20's anyday...
     
  6. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    really? i was always under the impression that it's better to run em at 23 for tread - keeps all the lugs on the ground rather than getting the middle worn out differently.
     
  7. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    nah... If anything they'll wear faster cause of sidewall roll causing the outers to wear faster...

    I suppose different brands may run differently depending on lug pattern, softness, etc... But I've ALWAYS gotten best tread life running 32... My BFG Mall Terrains have almost 55 g's on em and they're at about 35%, with perfect wear...

    I can feel the difference big time in my truck when I'm in the low 20's as opposed to 32...
     
  8. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    then i guess i'll try 32. :)
     
  9. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    What tires do you run?
     
  10. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    A 12.5" wide radial should be at least at 30 psi. Low 20's is for wider/taller bias ply's.

    At 2600 rpm cruise I did average 16.3 mpg to Moab and back...which is 100% freeway mileage. Any in town driving will pull those numbers down. You're probably losing at least 2 mpg to the additional rolling resistance due to not enough air in the tires IMO. You're also killing your tires driving with that little air. Heat kills tires and increases tread wear a lot...and low air causes so much more tread squirm and heat. Most blow outs on the highway are from running not enough air (intentionally or not)

    Rene
     
  11. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    Was hoping you'd chime in Rene....

    Yeah, I forgot to mention the heat issue...

    Isn't optimum mileage RPM for that motor like 2000 or slightly under that? Thats what I seem to recall...
     
  12. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    ok, so it looks like if i air up to 32 psi, and drive at 50 mph (2000 rpm), i'll really bump up the fuel economy.

    My truck was originially 6.2/700/10b/3.42. i guess that doesn't really matter to the question i'm about to ask, i don't think, since i now have a 205 tcase. anyway, can anyone estimate what my speedo would read at 50 mph? 60mph? i don't have a stop watch, so I'd have to pick one up to figure out speed in relationship to my speedo. I've been guessing recently. i suppose to get the proper ratio, it would be important to know which gear ratio was in the axles which were in the rig the tranny/tcase originally came from. hmmmm. guess i'll have to get a stop watch.
     
  13. cbbr

    cbbr 1 ton status GMOTM Winner

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    I lose at least 1mpg running the tires in te low 20's when I'm too lazy to air back up.
     
  14. imiceman44

    imiceman44 1 ton status

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    All you need is a budy and a cell phone.
    Have your buddy run at 65 in front of you and pace him and watch your odometer.
    that's how I did it.
    Also to know the best pressure for your tire:
    use chalk to make a straight even line accross your tire tread, drive straight for a couple of hundred feet and then check for the line wearing off.
    The best pressure is when the lone wears off evenly accross the tread.
    if it's over inflated the center will wear off, if it's underinflated the edges will wear off.
    It depends on your truck and the weight mostly.
    I run 36 front and 32 rear because I have a 1300LBS engine in the front.
    In other trucks I ran 30 front and 32 rear with a 350SBC
    Check with your tire and truck combo instead of guessing.
    :D
     
  15. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    The speedo head in the guage cluster is the same in all the 73-87's, so the only real variable aside from gears and tires is the speedo gears in the T-case. Seeing as I'm pretty familiar with that particular 205 I can make some educated guesses for you.

    With 4.56's and 39.5's it read 50 mph when it was doing 62 mph (actual speed determined with GPS). With 35's it read 56 mph when doing 62 mph.

    My best guess is that your speedo is saying 50 mph when you're really doing 62-63 mpg.

    Rene
     
  16. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    yes, optimum for fuel mileage is 1800 rpm at ~60-65 mph. With my NV4500 and the 39.5's my cruise rpm at 65 mph is 1840 rpm. :D Of course bias ply TSL's aren't the best when talking about rolling resistance (or ride, or roundness, or balance, or wear, or noise, or road manners...I could go on
    :doah: )

    I'm pretty much decided on swapping to some 325/85-16 Michellins which should still be great offroad, but wear better, are round and are quiet.

    Once I get a few more bugs worked out I'll get some freeway miles on the Jimmy and report. I'm hoping for 21mpg or better.

    Rene
     
  17. muddybuddy

    muddybuddy 3/4 ton status

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    well when my 6.2/4L80E swap is done, with the 5.13s in the axles and 39.5 im gona be a good bit over 2000rpm at 65. eventho 42's would give lower rpms, with the added mass, would they actually help mpg or would it end up being about the same?
     
  18. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'd guess you should still see 19 or so, maybe more...

    Rene
     
  19. kennyw

    kennyw N9PHW Premium Member

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    First of all you need to be able to correct the mileage in order to get an accurate MPG calculation. Long trips are great for this because you can use the mile markers on the freeway and compare the #'s on your odometer. If you drive 10 miles by the milemarkers but your speedo only turns 8.5 miles you can use that ratio on the speedo (100 miles will give you a more acurate # to work with though).
     
  20. kennyw

    kennyw N9PHW Premium Member

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    I used to get 19-21 mpg with my '79 K20 w/ 6.2L, SM465, NP205, 4.10 3/4 ton axles and 33" tires. Only down side was my top speed on the freeway :doah:
     

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