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2012 Sierra Crewcab 4x4 - 6" or 7" lift & gears questions

Discussion in 'Lifted & Offroad Suspension' started by Stickboy97, Jun 3, 2012.

  1. Stickboy97

    Stickboy97 New Member

    I just purchased a 2012 Sierra crewcab 4x4 5.3L w/ 3.42 gears and want to lift it. I know I can lift it 6" or 7" and run 33" tires with no issues. My question is if I want to go to 35" tires what gears do I need and why? To run 35" tires will a 6" lift be enough or do I need 7"?

    Just thought of 1 more question, if I go 6" or 7" what power adders can I add? Will I need them with or without new gears? How bad will my mpg suffer?

    I do mostly city driving but I do go to the deer lease a lot, hence needing the 4x4. Otherwise this all mostly just for looks.

    Thanks
    Shane
  2. the phantom

    the phantom Active Member 1000 Posts

    Well I can tell you from my experience with a 6" lift and 35.5" tires that you will want at least 4.10 gears. Although the Aftermarket company that put my lift and 35" tires on my truck says that you dont have to regear because it is with-in GM specs. I would still include the regearing in your budget if your going to put that size tire on. You can do some searching though and see that there are many dealers selling Rocky Ridge packages with the 6" lift and 35" tires with the stock 3.42 gears. I was not happy with the gearing and will probably changing mine soon. Im getting about 12 average mpg.
  3. SurrealOne

    SurrealOne Former Member 1000 Posts

    Regarding gear ratio:
    To get an idea of proper ratios you will need to first convert your new and old tire sizes from metric to inches. Here's a tool that'll help: http://www.miata.net/garage/tirecalc.html

    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:
    [​IMG]

    Obviously you can choose to lean more toward fuel economy, more toward power, or stay within stock ranges; the choice is yours. You can do more precise, Chevy-specific calculations, using tools here: http://www.4lo.com/calc/gearratio.php


    Why regear? Because your stock gears are intended to get (and keep) your stock wheels and tires rolling. If you increase the weight of your wheels and tires by adding heavier wheels and bigger tires then your stock gears will not be adequate to the task and you will add stress to the transmission. In effect it'll be like running your truck on a mild incline all the time -- as far as your transmission is concerned. If your gears are too long then you'll find your transmission hunting on inclines and downshifting 1 or 2 gears (dropping you out of overdrive at highway speeds) where it might not downshift at all if you have the proper gear ratio. That means in addition to increased tranny wear you are also hurting your MPG's due to a failure to use shorter gears for bigger tires.

    Will you need to trim? For 33's, probably not. For 35's, maybe, but trimming is very minor; someone with experience and a dremel can do it in 10 mins. Someone who has never done it before can do it in 30 mins if they apply themselves and are careful.

    I wouldn't recommend a lift height taller than 6" as the angles start to get pretty steep (read: increased wear) beyond 6". (You're already adding wear by lifting; keep that in mind.) Also, if you off-road I wouldn't recommend a wheel diameter larger than 18"; 20's mean low profile tire walls and those don't protect your wheel investment nearly as well while off-road as the higher profile tire walls with 18's and 17's.

    Other considerations:
    If you are running bigger tires then in a given distance your tires will rotate fewer times than they would with will smaller tires -- which is less opportunity for your brakes to do their job in stopping the vehicle. (i.e. You will need to increase your following distance because it'll take longer to stop your truck.) To compensate you should consider brake upgrades (improved rotors/pads or potentially a big brake kit, the latter of which will require 18" wheels at a minimum). Similarly your brake lines will be longer for a lifted truck, which will reduce pedal feel; upgraded lines (stainless lines, for example) can help restore this. A common complaint with lifts is the lack of ride quality with the shocks; you might plan for aftermarket shocks from ground zero if you care about your ride quality. Also, most large (35") tires are E-rated, which is fine if you tow, but if you don't then you might try to find a D-rated tire because it'll provider better ride quality than E-rated tires. You're also going to want a tuner to correct speedo error and to account for the regear and shift point changes after the regear; you might consider one that is capable of allowing you to do custom tunes (like Diablosport) and engaging a custom tuner (like Diablew, who goes by @06MonteSS on this site) to get the most out of your tune.

    One more thing -- if you're thinking 35's already ... and regearing ... then regear for what you ultimately want. You don't want to make changes for 33's and then do it all over for 35's.

    Surreal

    P.S. Obviously a truck is a system and when you lift one you have to correct many other things to bring the truck back to stock performance while lifted. Much of the necessary changes may have warranty impacts to you -- as in the dealership may not honour your warranty if you do them. You might want to consult with your dealership if the warranty is a concern for you.
    Last edited: Jun 3, 2012
  4. Stickboy97

    Stickboy97 New Member

    Thanks for the reply, that's a lot to digest & start planning for. Thanks for all the great info!
  5. SurrealOne

    SurrealOne Former Member 1000 Posts

    Happy to help. Keep us posted on your mods. I don't know about others but I love hearing what folks are doing to their trucks.
  6. Jeremy09LTZCrew

    Jeremy09LTZCrew New Member 1000 Posts

    On a NNBS truck you should be able to comfortably fit 35s with a 6 or 7". I would echo Surreal's statement about not going too high due to the angles. Additionally, the higher you go, the higher your center of gravity is, and if you're actually going to be using the 4x4 that's something you'll want to consider.

    If you really want to go with a 7", you can look at www.cognitomotorsports.com. They have a 7" for our trucks that claims to maintain the factory angles. However, the kit is also twice as expensive as you'll pay for BDS, Skyjacker, Fabtech, etc..

    If that link doesn't work, let me know. I'm doing it from memory.
  7. ahmitchell1

    ahmitchell1 New Member 1000 Posts

    There are guys on here running 35s with the rancho 4 inch kit which has a solid crossmember replacement. As everyone says re gearing is important with 35s
  8. SurrealOne

    SurrealOne Former Member 1000 Posts

    And even 33's...
  9. ahmitchell1

    ahmitchell1 New Member 1000 Posts


    My 3.73 Is great with 33s
  10. SurrealOne

    SurrealOne Former Member 1000 Posts

    I had OEM 3.73's and wasn't happy with them with the 33's that were on the truck when I got it.

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