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The Numbers Dont Lie GM..Increase in Tire size = Re-gearing The Diffs.

Discussion in 'Chevy Truck Talk & GM News' started by the phantom, May 4, 2012.

  1. the phantom

    the phantom Active Member 1000 Posts

    Was trying to get some ammo (research) together so when I take my truck to the dealer next Friday to convince them gm must regear the diffs for the 35.5 inch tires they sold me with my truck I found this..
    Gearing
    Problem:
    A major drawback to upping the tire size is getting power from your engine to the ground. Installing different-sized tires from stock will effectively change your truck's gear ratio. With a bigger tire, the truck will feel like it is geared higher (numerically lower), which is great for highway cruising, but not for low-end grunt, off-the-line acceleration, or passing power.
    Solution:
    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, divided by your old tire diameter, 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). For towing or performance, you would want the next gear ratio lower than the stock equivalent. So, if our '95 project Silverado had 30-inch tires stock, and a 3.73 axle ratio, that calculation would tell us we need a 4.103 (which rounds off to the readily available 4.10) gear ratio to get us back to stock. . And, keep in mind that 4x4s need both the front and rear diff re-geared to the same ratio


    So In my scenario my stock tire size being a 245/70R17 or 30.5 inches diameter, and the tires size they sold me with the truck new are a 325/60R20 or 35.5 inches and 3.42gears 35.5/30.5=1.1639344x3.42=3.9806556 so I think they should at least give me 4.10 gearing to return the engine performance to the ground.. Im gonna take what I found here with me. They cant argue whats in Black and White..

  2. AMac

    AMac New Member

    Very good information, thank you for the write-up. I'm definitely bookmarking for future reference. Good luck getting the dealer to re-gear the truck for you. When you say, "and the tires size they sold me with the truck new are a 325/60R20" is that because your truck is a special edition that GM put out? Doesn't make sense that they wouldn't think of re-gearing the truck when there will always be at least someone like who knows it needs to be re-geared.
  3. the phantom

    the phantom Active Member 1000 Posts

    Yes I bought the truck new from the dealer this way. This is the only thing im not happy with on the truck. I hope gm makes it right is all I can say...
  4. Coach24

    Coach24 New Member 1000 Posts

    Keep us informed and Good luck with GM.
  5. SurrealOne

    SurrealOne Former Member 1000 Posts

    I would like to know the dealership's responses.
  6. donl

    donl New Member

    Not a chance in hell.
  7. Dirty Dog

    Dirty Dog New Member Platinum Contributor

    The dealership I deal with will do almost anything to your truck to sell you the truck,,,, but disclaimer is signed before sale.... I have noticed a whole row of factory modded trucks just came in this past week, I might stop in and see if the dat sheet in the window has the proper gear ratio installed,for the tire size...
    I am going to guess yes just by the sticker price, compared to the normal stock version of the same trucks.......
  8. the phantom

    the phantom Active Member 1000 Posts

    I would also be interested if they regeared them.. Rocky Ridge apparently doesnt do it. I went to check on a 2012 model at a local dealership where I was working and the truck was gone in a few days. Didnt get a chance to check it out.
  9. Jimmeh

    Jimmeh New Member

    ^This. I highly doubt they'll regear it for you without you paying them to do it.
  10. O'Brien

    O'Brien New Member

    There are several of those 6" lifted setups at my local dealers as well. There's one dealer that only installs these 6" lift and 35 in. tires setups if the truck was originally equipped with a 6.2 and 3:73 rear end. Of course even that gearing won't compensate for the additional load of the 35 in. tires. The other dealers are selling some of the lifted setups on 4X4 CC 5.3 L engine with 3:42 rears. I can only imagine how doggy those setups must be on low end.These trucks are marked up dramatically in price as well.Like in the mid to low 50s. My son-in-law purchased a 2010 Tundra 4X4 CC almost two years ago with a factory approved Pro Comp 4" lift with pro comp rims and tires. He payed 45 and change for his. Of course the Tundra 4X4 came stock with 4:10 rear however Pro Comp installed a set of 4:30s as part of the lift package.The dealer even gave him his stock rims and tires back as they are listed on the invoice and weren't credited. His truck accellerates briskly down low and is set up proper for the lift and tires. Why GM cuts these newer 1/2 ton trucks off at 3:73 rears as options is beyond me.

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