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can I swap a pumpkin from a '91 2WD 3.71 14 bolt to a '85 14 bolt 4.1 for mileage?

Discussion in 'General Chevy & GM Tech Questions' started by KRCaddis, Aug 29, 2012.

  1. KRCaddis

    KRCaddis New Member

    Title pretty much says it all- I've got an '85 2WD C20 with a 14 bolt rear end and 4.10 ratio and I do highway driving almost exclusively. I'm getting 10-15 mpg at most with my 350 motor, AT. I'd like to do better as it's 250 miles RT each trip= expensive! I can get a 3.71 pumpkin from a newer (newer than '91) 14 bolt rear end, a 2500 2WD. Can I simply pull the axles, cover, input and swap out for mine? That way I think I can keep my housing, axles, brakes, etc and not move 500 pounds of iron back and forth from the wreckers.

    Anyone have experience with this enough to advise on fuel economy for highway driving? Wheels and tires are as original equipment, commercial tires on 16" rims. Tire pressure is as recommended (80 PSI rear)

    I did a similar thing on my other pickup, but I knew those to be interchangeable in advance and owned the parts already, anyway.
     
  2. KRCaddis

    KRCaddis New Member

    Good point everyone on low mileage over teh long haul, but gas here, now is $4.10+ a gallon and I'm driving 1000 -2000 miles a month moving, so I likely underestmated the total mileage by 2 or 3 times! I should have probably said" after I move" it's 10k miles in 10 years... Other option would be a tranny swap, but my former Chev 4x4, a 912500 went through 4 flex plates and 4 gearbox rebuilds in about 7 years, each time the rebuild warranty expired a flex pate would break around the bold circle or between that and the holes... Then the tranny would lose gears a month or so later Not too keen on the 700R4! None of the National chain and certified shops could give me any reasons and would say" this will fix that...
     
  3. Skippy

    Skippy Member 2 Years 100 Posts

  4. KRCaddis

    KRCaddis New Member

    KRCaddis again, to further clarify:
    My C20 rear end has 8 lug axles, does that mean it's full floating? It sounds like the 6 lug are semi-floating. How can I tell the difference?

    Other question - IF I can swap the 14 bolts as I propose (early 90's 14 bolt, 8 lug nto an 85 C2014 bolt 8 lug, can I simply pull the grear set and internals, which I presume controls backlash, clearances and is already set up if it comes out as a single unit (pumpkin), and replace it in-kind with the 90's unit?

    I don't want to buy the entire axle set up if I don't have to - they're 500 pounds and probably cost that much. My brakes, etc., are great - why change what's good with an unknown -higher mileage - truck? (gears last longer than brakes)
     
  5. Skippy

    Skippy Member 2 Years 100 Posts

    Man, I wish I had an answer for you. I'm a decent shade-tree mechanic, but never had an opportunity to have to do something like this. All of my experience is with the drive train in front of the transmission... Very little suspension and axle work.

    Hopefully, someone will chime in and give you personal knowledge of what you're trying to do...

    -Skippy
     
  6. KRCaddis

    KRCaddis New Member

    Thanks Skippy!
     

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