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Trying to determine axle ratio

Discussion in 'Chevy Trucks' started by gamfa, Sep 7, 2003.

  1. gamfa

    gamfa Junior Member
    Messages: 20

    I bought a truck last week that I want to fix up as my farm truck and I have a question or two (there will be many in the future). This pup has an "assembled from parts" title but thanks to Chuck's truck site I identified most of the parts. So far I know the following... 1 ton dually rear, NP 205 transfer case, 4 speed trans. that I believe to be the overdrive unit, Dana 44 front axle and 6.2lL diesel engine. It has a GMC 1500 cab of unknown year that's in pretty good shape. The doors on the cab are from another year and have electric windows and locks. The truck has no air conditioner. The date on the engine air cleaner is May 1982 . The wiring is a mess but the engine runs. I am trying to determine the axle the ratios. I thought I could count the number of rotations on the drive shaft to one rotation of the wheel and that would give it to me directly but that must not be the case. On the rear I get 1.86 rev's. of the drive shaft to one rev. on the wheel. The transfer case is in 2H and transmission in neutral. With this info can anyone determine the axle ratio? The second question is, can the Dana front end be re-geared to match any standard rear ratio? I am assuming currently that the rear and front do not match because the front drive shaft is missing.

    Thanks...
     
  2. FSUPERDUTY

    FSUPERDUTY Senior Member
    Messages: 311

    I doubt the trans is an overdrive.the axle ratio is probably a 4.10 with 3.73 and 4.56 being very possible hope this helps you!
     
  3. Pelican

    Pelican 2000 Club Member
    Messages: 2,075

    To determine your gear ratio, jack the rear of the truck up and block it, mark one rear tire at the bottom with chalk. Make an inline mark on the driveshaft and pinion housing for reference. Now make one full revolution of the tire and count the number of times the driveshaft turns. 3 3/4 turns = 3.73, just a little past 4 = 4.10, 4 1/2 = 4.56, etc.
     
  4. wyldman

    wyldman Member
    Messages: 3,265

    Your better off turning the driveshaft,instead of the wheel when counting turns.If you turn the wheel,and it's an open rear end,then the spider gears may turn and throw off your readings.This can happen when turning the driveshaft too,but it's less likely.

    Sounds like thats what happened when you tried to determine the gear ratio.The spider gears will give you a 2:1 ratio when turning so you can still figure it out.

    If you do the math,1.86 ratio measured X 2:1 for the spider gears gives you an effective ratio of 3.72.So it's probably a 3.73.You can double check by getting someone to help turn BOTH wheels while counting.

    You can check the front too,and see if it matches.It's easy to regear if neccesary.
     
  5. gamfa

    gamfa Junior Member
    Messages: 20

    Thanks wyldman. I'll try it from the drive shaft before I go to the field in the morning and see how it matches up. It is an open rear end.

    One thing I've been wondering about is if the Dana can be modified to take Bud wheels? The rearend has Buds but the front has standard 8 bolt wheels.

    I don't really know about the trans. except that it is a 4 speed. The mention of overdrive come from the fact that there is an actuating cable mounted in the transmission hump a little behind the transfer case lever and to the left. The cable goes to a housing on the left side of the case. Not knowing anything about truck overdrive transmissios of that era, I thought it might be an overdrive unit. The underside has a very thick coating of grease, oil, and desiel fuel on it. I'll pull the power washer over to it and clean it off in the morning and see if I can make any descovery of what it is.
     
  6. Pelican

    Pelican 2000 Club Member
    Messages: 2,075

    Oops!

    In my first read, I missed where you had already tried my process:eek:

    You'll need the adapters to mount the Budds up front, they're just a spacer that makes up for the extra offset and bolt on in place of the studs.
     
  7. wyldman

    wyldman Member
    Messages: 3,265

    That cable would be for the speedo,not an overdrive unit.
     
  8. gamfa

    gamfa Junior Member
    Messages: 20

    After cleaning it up and looking real close, it appears to be a PTO unit. Do those things mount on the side of transmissions. Never had one and don't know anything about them but that is what it looks like. There is a short stub shaft that sticks out to the rear that I can turn by hand.

    I was able to turn both wheels myself and I got about 3.75 on the driveshaft. I didn't go to the extreme of measuring the actual distance on the circumference of the driveshaft like I did the first time but it was close enough to figure it's a 3.73. I will try the front axle when I get the time.

    Pelican...can the Budd adapters be bought from GM or do I need to find some sort of wheel shop that sells them?

    These are things in the future. I have to spend what time I can on the electrical system now. It's a total mess. I really need some decent wiring diagrams on the cab and engine systems.
     
  9. wyldman

    wyldman Member
    Messages: 3,265

    Sounds like it probably is a PTO.Not common on pickups,but they were used sometimes.Maybe for a dump hoist ?


    I have a lot of the older wiring diagrams,but they are old,and dirty.I know I can't post them here,because at the low resolution,they are too blurry.I could scan and e-mail if you let me know what all you need,or I could just walk you through it here.

    The wheel adapters can be found in the junkyards.
     
  10. gamfa

    gamfa Junior Member
    Messages: 20

    Thanks for the offer wyldman. I may take you up on it in the near future. I bought the Haynes manual on the truck and I already had their manual on diesel engines since I had to trouble shoot the glow plug sys. on my 88 F250 7.3L engine a few years back. I also have their manual on Fords 1980 thru 1987 for the 1980 F250 I'm using now as my farm truck. The wiring info on the Chevys isn't near as good as the manuals on the Fords. I'm hoping to find one like they have on my 80 that is one large diagram that covers the truck from bumper to bumper with everything shown on one diagram instead of systems taken out of context with the rest of the truck. I have ordered the Chilton's manual in hopes that their diagrams are better. It will be a few weeks till I have much time to spend on the Chevy. This is a pretty busy season with cutting the last stand of hay and marketing the calves. I also do contract baling and contract hauling for other producers. Got to keep the Super Duty on the road so it can pay its way as much as possible.
     
  11. wyldman

    wyldman Member
    Messages: 3,265

    It's hard to find good wiring diagrams,even I don't have them all.I can draw alot out from memory,as I know where most of the stuff hooks up anyways.If you need me,just let me know.