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Couple Questions: Gearing, MPG & AC

Discussion in 'Chevy Trucks' started by fedspunisher, Aug 12, 2008.

  1. fedspunisher

    fedspunisher Member
    from Iowa
    Messages: 89

    I have a 1980 Chevy 1/2 4x4. The gearing is 2.56 I found this out after trying to pull 8000lbs home the other day. Wondering why it was such a dog I jacked it up and did the wheel to axle rotation. Im looking to change this to something from a 3.73 to a 4.56 but im not sure which.

    It is my daily driver and would be nice to have better MPG but would also like it to tow and plow. With the 2.56's it doesnt even get 10mpg on the high way. It has 31's on it with a 350 and a TH350. Wondering if the 2.56 were out of power band causing excess fuel comsumption or if something else is wrong.

    It came with AC and everything is hooked up and was under the impression it worked when i bought it. It runs and everything seems fine i checked the pressure and it checks good while the compressor is running but when its not the pressure will climb to about 125psi. Also while on max ac the compressor will only run for about 2 seconds and then shuts off and cycles back on with this same pattern. No cool air is to be found. Any help on the above would be appreciated!

    Thanks!
     
    Last edited: Aug 12, 2008
  2. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    What was the exact procedure you used to check the gear ratio feds?

    I've NEVER seen a gear as high as 2.56's in a solid axle 4X4 GM.

    Of course someone could have swapped them in sometime in the past.
     
  3. fedspunisher

    fedspunisher Member
    from Iowa
    Messages: 89

    When i bought it this is what the guy said was in it i then asked and people doubted it but after i pulled the trailer i was pretty positive the guy was right. I held one wheel turned the drive shaft until the tire made one revolution. I got about 1 1/4 turns on the shaft then you mulitply that by 2 right? and that put me around 2.56
     
  4. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Yes your calculations and checking procedure are correct....no wonder it's a pig.

    A set of 3.42's would wake it up immensely. :nod:
     
  5. fedspunisher

    fedspunisher Member
    from Iowa
    Messages: 89

    Do you think the 2.56's would hender the mpg? My dad had a 77 K5 with a 400/400 Full time with 33's and got 16 on the highway. I cant even get 10
     
  6. William B.

    William B. Senior Member
    from S.E. IA
    Messages: 978

    I had 2.73s in my 80 K-10. What size tires are you running?
     
  7. fedspunisher

    fedspunisher Member
    from Iowa
    Messages: 89

    31x10.5.......
     
  8. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    With 31's it definitely could. And with your current tire/gear combination you'd need to run at a min of 70 mph in order to have the engine even near its "sweet spot". Which is where your mpg's will be the best.

    And of course theirs many other factors that affect the mpg of a truck including but not limited to a good state of tune, properly working/tuned carburetor, ignition timing, road speed, tire design, and driver habits all affect mpg.
     
  9. 78K20

    78K20 Member
    from WV
    Messages: 49

    I had a 78K20 with a built 350, TH400, 4.10 gears and 33s, best i ever saw was 12mpg, but if i were you i would atleast go for 3.42 IMO maybe 3.73 if you are gonna tow, worst mileage i got towing a 7k load was 7 to 8 but i had plenty of power
     
  10. fedspunisher

    fedspunisher Member
    from Iowa
    Messages: 89

    I believe its a stock 350 out of a full size van with a quadrajet. Not sure of the year.

    I heard that the cases are either for 2.56 and down or 2.73 and up so if i do have 2.56 i could only go down with out changing the case????
     
  11. fedspunisher

    fedspunisher Member
    from Iowa
    Messages: 89

    Im hoping that it has 2.73 in it and i just screwed up figuring out what it has..
     
  12. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    As long as the axles haven't been changed by a previous owners sometime over the years your front axle should a a 10 bolt 8.5. Which only has one carrier case break- which is 2.73-up. And 2.56-down.

    Your rear axle should be a 8.875 12 bolt which has three case breaks: 2.56-2.92, 3.08-3.42, and 3.73-up.

    Finding the correct series carrier for either shouldn't be too hard...or you could kill two birds with one stone and replace both carriers (or at least the rear) with some nice aftermarket limited slips or full lockers since it is a plow truck after all.

    May want to double check your ratio before doing anything...

    And don't expect too great of mileage no matter what you do with gear ratios. :)
     
  13. fedspunisher

    fedspunisher Member
    from Iowa
    Messages: 89

    Thanks for all the help. This is the bad thing about buying a modified truck. I know he said something about the rear locker and the fact he put synthtic oil in it. Thing does lock up super easy just giving it gas around a corner (just normal driving not hot rodding) is enough to lock up the rear axle. Im sure it'll be a blast this winter.
     
  14. scott nadeau

    scott nadeau Junior Member
    Messages: 7

    You definitely want to stay in the 3.xx range - not into the 4.xx range. 4.xx will kill your milage on the highway - it will be revving to the moon.

    3.42 would be good for milage, 3.73 would be better for towing.