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gas milage

Discussion in 'The Garage' started by bigbadchev84, Sep 13, 2006.

  1. bigbadchev84

    bigbadchev84 1/2 ton status

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    heres the specs:

    78 chevy 3/4 ton

    290hp gm crate motor with edelbrock performer intake, 625cfm carter afb, headers and stock hei

    sm465/208

    4.56 gears and 35x15.5x16.5 tsl sx tires


    what can be done to improve on the 6-8mpg that its currently getting?

    new 35x12.5x16.5 bfg m/ts will be purchased soon and a new carb will be purchased in the very near future also, but what would be the best choice for this combo? it is mostly street driven and is a daily driver, my brother just wrecked his car and is now commuting in his truck until he can get the car paid off and get a new one.

    would a new fuel injection kit make that big of a difference like the ones jegs and summit sells?
     
  2. longbedder

    longbedder 1/2 ton status Premium Member

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    Only drive downhill with the engine off. Or dump all the money you're gonna pay to eek out 12 mpg from that thing and buy some used Honda.

    Besides being properly tuned, there's not a lot you can really do on a rig like that. A '78 3/4-ton with low gears gets straight-up awful mileage.

    FI would help a bit. An overdrive tranny would help a bit. Any gears taller than 4.56's would help.

    Before you make any high-dollar changes, you should do the math on increased fuel mileage versus how much the changes cost you up front. This will be a function of mileage difference with the changes, mileage driven per year, the cost of the mods, and the cost of fuel.

    I don't know what axles he's running, but switching to 3.73 gears would be one of the most economical first moves.
     
  3. Goose

    Goose 1/2 ton status

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    Also is his speedometer accurate, and therefore are his mileage figures accurate.

    I keep on hearing how people loose gas mileage when going from a Q-jet to any other carb. If he's not running one, that may be a lot cheaper than FI and almost as good as mileage if tuned properly.

    I could get 12mpg on the highway with my 350/np205 combo with 4.11 gears and 31.6" tires which equal the same final gear ratio. My motor was stock with a Q-jet.

    I know once I got about 60-65mph my mileage would drop to about 9-10mph
     
  4. muddybuddy

    muddybuddy 3/4 ton status

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    swapping in a diesel like im going to be doing ;)

    TBI would help you a bunch....my current setup is 350tbi/700r4/208 5.13s and 39.5's and im getting about 8 city 11 highway. if you went with TBI i would think you could be getting 10-12 city, and 14-15 highway.
     
  5. k5floyd

    k5floyd 1/2 ton status

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    4.10s would help and if ya got the dough a OD tranny and/or FI set up would would help a bit
     
  6. blazinzuk

    blazinzuk Buzzbox voodoo Premium Member GMOTM Winner

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    The tires will help some. May I suggest a quadrjet when you get the new carb if you stay out of the secondaries a qaud will get better mileage than any other carb (tuning being equal). I think electrical upgrades would be in order. new coil and module for the HEI make sure the plugs are in good shape and the plug wires. If I were to go fuel injection on it I would junkyard a TBI setup that will help some but a properly tuned carb should be very close to a TBI on fuel milage. I think you should be getting better than that. My setup is a crate 350(the cheap one) turbo 350 trans 203 case and 1 tons w 4.56s and 40" IROKs I get 9 mpg on the freeway if I stay at 65. Mabey just slow down a bit, at 75 I get 7 mpg
     
  7. 1977k5

    1977k5 3/4 ton status Vendor

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    I think most of your problem lies in the mistuning of your aftermarket carb and/or your low gears with your 35" tires. There are too many people that ditch the Q-jet for something else because they think it is difficult to tune when it is probably the best carb ever made. I have 4.56's with 37's, a 465 4 speed, and a 430 hp 383 with a Q-jet and still average 12-13 mpg (almost 15 highway). To me, I would bet the carb is tuned wrong.
     

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