How to get right gears for my tires and adjust transmission

Discussion in 'GM Powertrain' started by b-radical, Jun 3, 2012.

  1. b-radical

    b-radical Rockstar 100 Posts

    I have been running my 265/70/16 tires on my silverado since I got it. The truck is supposed to have 235/75/16 tires. I would like to change the gears in the diff to fit the tires I have, and put in a g80 rear locker at the same time. How do I go about finding the right gears?

    I would also like to adjust the transmission to run properly with the bigger tires. I think the shift point would be adjusted but I'm not sure.

    Can anyone help with this?
  2. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    Regarding gear ratio:
    You'll need a hand-held programmer that's capable of adjusting gear ratio size so that after you've got the new gears into the vehicle you aren't throwing error codes/idiot lights. You might as well make sure that tuner will also allow you to adjust your shift points. You can do it, yourself, or if the tuner you get is a Diablosport then you can work with someone like Diablew (a site sponsor, here) to get a custom tune. Lew sells bundles (i.e. device + custom tune) and his pricing is very reasonable for his services ... especially if bundled, so you might reach out to him if interested. He goes by [MENTION=32828]06MonteSS[/MENTION], here.

    Now, as for the gear ratio choice, you will need to convert your new and old tire sizes from metric to inches. Here's a tool that'll help:

    From there you can eyeball the proper ratio using a Jeep chart like this one, with the understanding that this chart assumes a manual transmission with a 1:1 ratio (i.e. no overdrive engaged) at 65mph:

    You can do more precise, Chevy-specific calculations, using tools here:
  3. Sierraowner5.3

    Sierraowner5.3 Epic Member 5+ Years 1000 Posts

    thats not that much of a tire size change between the two sizes. dont know which axle you have, but i would see if i could find a axle in a junk yard that had close the right gear ratio, correct the speedo, and be done with it. to put a set of custom gears in and buy a G80 wouldnt be cheap.

    I cant see a junkyard axle costing much more then $250, and if you find the right one, it should bolt right in, dont know how mechanically inclined you are, but it is something u could do in your driveway.

  4. the phantom

    the phantom Epic Member 5+ Years ROTM Winner 1000 Posts

    Yeah sierra is right. Your diameter difference is only like 7/10 of an inch larger on the 265 tires. its only throwing your spedometer off like 2mph at 65. Here is the formula though to figure out your gearing for different size tires.
    To bring your truck back to stock performance, it is important to re-gear the truck accordingly. A simple calculation will tell you what gear ratio would get you back to your stock equivalent. The calculation is your new tire diameter in inches, divided by your old tire diameter in inches, multiplied by your old axle ratio, will equal your new axle ratio (new tire diameter/old tire diameter x current axle ratio = new axle ratio).

    Your stock tire diameter is 29.9 inches and your larger tire diameter is 30.6 inches. Not sure what gear ratio you have but I think this info will tell you what you need to know.
  5. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    I personally wouldn't regear for that small a change in tire sizes, either. But hey, some people do...
  6. b-radical

    b-radical Rockstar 100 Posts

    Thanks for all the good feedback guys. I don't think ill be going through with the gear change considering all the suggestions. However, I will be putting in a G80. I vaguely remember a link for a website than allows you to put in your vin # and get all the stock equipment your truck came with. Does anyone have that link?

    Thanks again,
  7. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    Last edited: Jun 8, 2012
  8. b-radical

    b-radical Rockstar 100 Posts

    sweet :great:, Thanks!

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