differential ratio

Discussion in 'GM Powertrain' started by murph350, Nov 3, 2012.

  1. murph350

    murph350 New Member

    I have a 1990 gmc sierra 4x4 ,Ive owned it a couple of years and afert having the transmission and transfercase rebuilt I just put it in 4wd the other day for the first time and realized my front and rear differential are different and the rear is pushing the front I need to find out the front end ratio for that model since changing threar would be easier than the front
  2. donyms

    donyms Epic Member 5+ Years ROTM Winner 1000 Posts

    Welcome to the club, it's good to have you aboard. If you post this in the tech portion of the site you may get your answer quicker. Good luck. :glasses:
  3. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    Moved to powertrain forum.

    If it's apparent that someone has changed one of the gears, then I would think the only way to be sure of the front gear ratio is to actually count it out, unless you have some good service records for the truck. Sure, you could check the RPO code in the glove box, but there is likely no way to know if that is still what is in the front end of the truck.

    To measure the gear ratio, jack up at both front wheels and at least one rear wheel. Put the transfer case in 4H and the transmission in N (with the stock thermoelectric center axle disconnect CAD, you will probably have to have the key on for this test, to make sure the CAD is engaged). Rotate that front tires through one rotation (make sure both tires turn together) while counting the number of rotations of the front driveshaft. The number of rotations of the front driveshaft is your gear ratio (about 3.5 rotations means 3.55 gear ratio).
  4. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Reading [MENTION=11846]MrShorty[/MENTION] 's answer, has me thinking; if you jacked up all four, marked the bottom of each tire, and started the engine, placed the trans into a forward gear and let the wheels turn a couple of times. you could be sure the gear ratios are different if the tire marks are no longer together (say, both are not at 3 o'clock).

    Note, you could do this with only one side in the air if you are sure the rear is not posi/locking

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