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School me on changing out transmission gears - tires are too big

Discussion in 'GM Powertrain' started by McClintoc, May 1, 2013.

  1. McClintoc

    McClintoc Super Moderator Staff Member Platinum Contributor 1000 Posts 100 Posts

    I just realized my tires are way too big. My last set of tires were 265-65-16s on OE wheels. Those tires were already over-sized from the 235s that came on the truck. This past December I got some new 17" wheels and four new tires to go on them. My new tires are 265-70-17s. Therefore, I have increased both my wheel diamater and my sidewall height. These new tires are almost 3 inches bigger in overall diameter than what came stock. Thinking back on it, I should have gotten some 265-60-17s (which don't seem to exist) or maybe some 255-65-17s. Either way, instead of decreasing my sidewall height to compensate for a larger wheel, I increased my sidewall height and now my tires are way too tall for my stock transmission.

    First off, is my above thinking accurate?

    Secondly, how much does it usually run to have the gears changed out? I just had the trans rebuilt about a year ago after it died on me (roughly 130k miles) and that was to the tune of $2200. I'd have to have a shop do it as I do not have the tools nor the knowledge to do it myself, unfortunately. I know the cheapest route would be to exchange my tires for a smaller size but the ideal size I want (265-60-17) doesn't seem to exist.

    Thanks for any input!
  2. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts 100 Posts

    First of all, you don't change transmission gears to compensate for tire size. All transmissions I can think of have set gear ratios with no ability to alter the ratios. What you want to do is put taller gears in the differentials. If you have 4WD, you have two sets of gears to swap out. Running rate of gear changes by reputable shops I've seen run in the 500-800 range per axle, so if you have a 4WD truck, double the cost because the gear ratios have to match front and rear.
    Last edited: May 1, 2013
  3. McClintoc

    McClintoc Super Moderator Staff Member Platinum Contributor 1000 Posts 100 Posts

    Good thing I asked, as I was talking out of my rear end! (Pun not intentional but there it is, lol)

    That's actually good that it's the rear diff then as my rear diff fluid has not been changed in quite a while. Sounds like I'll find a good shop to change the gears and I'll have them change the fluid and probably all the seals. Just completely rebuild/upgrade the rear diff in one stop.

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