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 New Member

    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..
    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.
    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 New Member

    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

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

    SurrealOne Former Member

    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

    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 New Member

    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.
  11. the phantom

    the phantom New Member

    Yeah I havent even heard of any having the 3:73's Hopefully I can convince the dealer (not the one I bought it from) to advise gm on how doggy mine is and that its not right.. by reverse math of my original post if my stock gearing was 3:42's then after adding 35.5" tires my gearing or power to the ground became like having 2:94's. I just hope I dont hear from whoever may become responible that "oh thats not much less than having 3:08's"
  12. tbplus10

    tbplus10 Super Moderator Staff Member

    How were the tires and wheels listed when you bought the truck?
    If their listed as a dealer add on or a regional add on package, which I'm betting they are, GM wont touch the issue, why should they, they warrantied the truck with the tire size it left the factory with, any different tire size that was on it when you bought it is between you and the dealership, new or used wont matter.
    Did the dealer specify a warranty for the tires or add-on package they may have been part of, probably not but you should be checking your paperwork for a warranty.
    This tire issue isnt GM's problem, I know thats not what you want to hear but it's the truth, even The Magnussen Moss Warranty Act wont cover you here, the tires were changed after they left the factory, either at the dealership or a regional distributer and GM wont assume responsability for change made to the vehicle after it left the factory.
    Dealer add-ons arent covered by GM warranties unless specifically included in the warranty, and I know theres very few items that are allowed to be included.
    Only tire sizes on GM's list can be covered by warranties, 35's arent on the size list. New gears would be a stretch since the trucks computer is programmed for certain parameters and tire sizes, 35" tires arent in those parameters or sizes.
    As a matter of fact I believe there was a poster in Virginia about 6 months ago that tried to get work done on his truck he'd bought with 35" tires new from a dealer and they informed him even though he bought the truck new from a dealer with those tires the warranty was voided because of them. Read up on Magnussen Moss Warranty Act because I know GM is gonna know all about it and quote it to you.
    It's nice that you found a website that says you should change gear ratios when upsizing tires, but it isnt an officile GM site so it means nothing to them and again the factory more than likely didnt add those tires.
  13. the phantom

    the phantom New Member

    I was assured by the dealer I bought the truck from and a couple other dealers that sold these trucks I talked to before I bought mine that the warranty covers the entire package including rims, tires, lift, drivetrain ect. 5year 100000 mile warranty on drivetrain. It states this on the window sticker of my truck so Im not concerned wether its covered or not. Its a matter of wether they feel that it should be changed. The computer is changed for the size tires on my truck. The spedometer has been corrected and the shift points have been changed. But that cannot makup for the power to the ground gearing.
    Last edited: May 6, 2012
  14. O'Brien

    O'Brien New Member

    You're covered. Dealers install factory approved kits on these new units. I already asked. Problem is they ain't geared right for low end or engine longevity. The trade off to them(the factory) is the inflated price that the dealer must work out between he and GM.Otherwise, I think you're on the right track. If it were mine I'd want it setup properly.Problem is ...ultimately you'll have to work through the dealer.As an example,if you wanted to install GMs current 373 setup it wouldn't fit in your rear case anyway as it's 9.5 in ring gear. The issue becomes an aftermarket set of 4.10s or the like that only an authorized dealer can get the go ahead on. If you were to have it installed by an independent shop yourself it would indeed void the warranty. I've been down that road already. I'd call both GM and the dealer and discuss the matter with a level head and see if something can be worked out. If not you may even entertain some sort of buy back proposition if you're displeased.And I'm guessing you are. I'd be interested in seeing how this turns out. Good luck.
    Last edited: May 6, 2012
  15. tbplus10

    tbplus10 Super Moderator Staff Member

    And again 35" tires arent on GM's approved tire listing so I dont know how a dealer could warranty it from the factory, dealers cant make factory or GM warranty promises written or otherwise where it concerns modified or converted vehicles.
    Any warranty promise they make would come from the dealer or conversion company and would need to be backed in writing by an independent warranty company.
    As I said changing tire size to a larger tire not on the GM size approved list would void GM's warranty and responsibility.
    Factory GM programming for the trucks computer doesnt have an option for 35" tires because GM recommends you dont put tires that large on their 1500 series trucks.
    I dont doubt the tires make the truck feel powerless but it really isnt GM's problem.
    The "Rocky Ridge" package is a regional package put on the truck after the factory, its sold by Rocky Ridge Conversions of Franklin, Ga. and since it is a conversion package if you do your research I believe you'll find very little of your factory warranty is still intact.
    You really should read the Magnusen Moss Warranty Act before you go in with your guns blazing because by virtue of your truck being a conversion your warranty has been drastically altered and you may not have a correct understanding of what rights and responsibilities are involved with your truck regarding this particular issue.
    If you read the Rocky Ridge website they never claim to be a Factory approved conversion company, they do though offer 3yr/36000 mile warranty on their conversion packages.
    Why would they offer that if GM was still covering the truck?
    Last edited: May 6, 2012
  16. ahmitchell1

    ahmitchell1 New Member

    I'm at my lake house in Franklin ga. I'm very close to rocky ridge conversions, I very interested in this. I might go over there tomorrow and ask them bout all this


    I would love to here how this turns out. Those Rocky Ridge set-up trucks sure look great but apparently looks can be deceiving. They have some of them at my local dealer I'm going to check them to see what set-up they have in them.
  18. catmechanic07

    catmechanic07 Former Member

    I agree with donl. GM is not the ones installing these lifts when they build the trucks. You cannot order a truck from the factory with a lift AT ALL. The dealer you purchased the truck from has installed a lift and as far as warranty.....the dealer that sold you the truck honors the warranty AND extended warranty on their behalf.......not GM in MI. If they do honor your warranty and regear for you.....I wanna know your dealers info and Ill take my truck there for all my warranty work.
  19. davedyno

    davedyno New Member

    Gears & Tire Size

    I agree, your info is correct. But if your speedo reading is correct, I don't think they will do anything for you. If your truck is a 4X4 front and rear gears would have to be replaced,cost would be $$$$
    2011 Silverado Ext. 5.3 4X4
    18" wheels
  20. the phantom

    the phantom New Member

    Ive already been told by the dealer im taking my truck to (not the one I bought it from) that the entire truck is covered under warranty. Who is covering it I dont know and really dont care. I know its not the dealer I bought it from but its either GM or Rocky Ridge. I just need to have the dealer im taking it to convince whoever is covering the vehicle to make the gear change. I just stopped at a dealer that had a brand new 2012 gmc sierra sle crewcab with 3.42 gears had 35" mt tires on it with the rocky ridge package and that was listed for $59,995. It also listed that it had the full 5yr 100000 mile warranty on the entire truck with rocky ridge package. So warranty is not my concern. Obviously a deal had been made between GM and Rocky Ridge on these packages.

    ---------- Post added at 06:39 PM ---------- Previous post was at 06:16 PM ----------

    I found this advertised on a dealers website for a rocky ridge truck [​IMG] Stock Number: 12205

    2012 Silverado 2500HD Crew Cab Standard Box 4-Wheel Drive LTZ
    Lift Truck #12205*
    CAUTION: Other aftermarket lifts void your manufacturer warranty! Rocky Ridge is one of only two GM approved up-fitters - that means that your lift truck is still covered by all manufacturer warranties! Do not put yourself at risk, there is nothing else that compares to a Rocky Ridge Conversion. Rocky Ridge custom vehicles are new and have not been titled.
    Last edited: May 8, 2012

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