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Front to rear end gear ratios

Discussion in 'The Garage' started by Phatsalmon, May 1, 2005.

  1. Phatsalmon

    Phatsalmon Registered Member

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    Guys,

    I am having a problem with my front tires "dragging" my rear tires in 4 wheel drive. I have broken u joints as well as a drive shaft. My question is "are the front and rear end gear ratios on a 1978 Jimmy the same?" Mine are not. It looks as though this may be my problem. I have a 41 tooth ring on the back with an 11 tooth pinion. On the front is a 40 tooth ring with a 13 tooth pinion. One other question, how critical is tire size. My front to back tire diameter is only 1/4 to 1/2 inch difference. Please can anyone help?
     
  2. R72K5

    R72K5 Banned

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    uhmmmm you have 3.07 in one and 3.73 in the other,..


    you have MAJOR problems there,,,,

    you better match the axle ratios! lol

    or kaboom will go the t case, tranny, or driveline,

    thats all i can say


    welcome to CK5

    good luck !
     
  3. Eric M.

    Eric M. 1/2 ton status

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    Tire size is pretty critical too. It's best if all tires are the same (brand, make and size). If not, get the actual specs from the manufactures and make sure the height is very close. You'll know if you're too far off.

    Eric M.
     
  4. Phatsalmon

    Phatsalmon Registered Member

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    Trouble

    Is it normal to have two different gear ratios between the front and back? The difference might be compensated for in the transfer case. My tires are the same, they are just 1/4 to 1/2 inches differenc in diameter. I am so lost in this right now.
     
  5. Phatsalmon

    Phatsalmon Registered Member

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    Help

    Tires are the same. Just 1/4 to 1/2 inch difference in diameter. I am not sure how to find out if the difference in gear ratios is normal and just being compensated for in the transfer case.
     
  6. speedyvision917

    speedyvision917 1/2 ton status

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    they shouldnt be different and if they are you should definately get yourself a new set of gears, but look on the bright side, now you can go out and pick out the gears you want and tires too if you need them :saweet:


    oh ya and...






    WELCOME TO CK5!
     
  7. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'd guess the previous owner blew the rear end and swapped in whatever was handy...then failed to mention you wouldn't be able to use 4WD until you addressed the problem.

    Slight variations in tire diameter won't hurt anything. Commonly the front and rear gears are always slightly different. 3.07 fronts and 3.08 rears, 4.09 fronts with 4.10 or 4.11 rears, 4.57 fronts and 4.56 rears.

    All those are acceptable.

    Looks like you'll need to find a 3.73 set for the front.

    Rene
     
  8. diesel4me

    diesel4me 1 ton status Premium Member

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    I had the same scenario..

    I bought a 71 K5 for 300 bucks years ago,and the first time I put it in 4wd a few weeks later and locked the hubs I found out why the price was so low!--the previous owner had put a 3.08 rear end from a 2wd C10 in it when the 3.73 ratio original one blew..and he "forgot" to tell me.. :mad:--but it was a decent truck otherwise,so I just took my lumps...

    I got another 3.73 rear end eventually,but I learned in the meantime I could put larger tires on the front,and smaller rear tires to compensate for the difference in ratios--I had 31x10.5x15's in the front,and 235/15's in the rear,that are about 28" in diameter roughly--it looked goofy as hell,but I was able to use 4wd and go sandpit wheeling with my friends all summer that way--I eventually sold it and just gave the rear end to the new owner--he moved to Texas and never installed it--his father called me and said to come get it if I wanted it--10 years later!--finally sold it to someone else.. :crazy:
     
  9. txfiremank5

    txfiremank5 1/2 ton status

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    Like they mentioned, it's not "normal" to have different ratios... and they didn't come that way, it was swapped in. Like tRustyk5 said, the only difference is very small, like 4.10 vs 4.11.

    As for being compensated for by the T-case... trust me, you don't want that compensation.

    Compensation usually happens on the ground... with tire slipage.. until too much traction is gained, and then it finds the next weakest spot to tranfer to... driveline components.

    You need to get new gears for one of the axles, so that they match. I would suggest getting another set of 3.73's for now. Unless you would just like to do a whole (ft & bk) re-gear now, and go with a ratio of your choice.

    A cheaper and somewhat easier choice is to find another 10 bolt rear with 3.07's and swap it in... but you'll have 3.07's, and be able to wheel, if nothing else.

    Good luck
     
  10. Phatsalmon

    Phatsalmon Registered Member

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    Thanks Fire Man

    Fireman,

    Things are coming together here. I did by the Jimmy about 5 years ago. At that point in time it did have two different sets of tires on the vehicle. I replaced those tires with 31 15 11.5 tires and this is when the nightmare started. I also noted a remarkable change in the speedometer at this time.

    Now about ratio's. I am thinking about mileage. I am getting only around 11 miles to the gallon. I would like to put in higher gears. If I need the lower gears I can use the low range on the transfercase.

    Any suggestions?
     
  11. txfiremank5

    txfiremank5 1/2 ton status

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    I would keep the 3.73's with the 31's. It won't change your mileage, but it will give you a very good all around range.
     

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