Running 38s with a 6inch rough country kit on my 07 2500HD chevy

Discussion in 'Chevy Truck Talk & GM News' started by Mud_Slut7, Oct 22, 2013.

  1. Mud_Slut7

    Mud_Slut7 New Member

    Hey all,
    new to the site. I have a 07 2500HD Silverado. Its a 6.0/gas engine and I'm pretty sure, stock, I'm running 4.10 gears. In the next week I'm having a 6 inch rough country installed along with 38x13.5R17 tires. Probably 9inch wide rim. The guy at the shop told me they would fit with some trimming and I'm pretty sure he said like 4.5-4.5 backspacing (as in equal on both sides). He's an old guy who's been doing this kinda stuff for years so i trust him, but at the same time I'm kinda erie about it because of all the people who say they won't fit. I already know that pretty soon after ill be looking at rehearing probably to 4.88s. What gears would give me the best fuel economy? Don't tow ever really. You guys have any insight on this project? Any tips?
  2. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    You might consider a body lift also. That would most likely eliminate the need for any trimming. You can install it yourself or have someone do it. It should cost you under $500 installed. I think that the kit is around $250. For your ratio you use this formula:

    rpm = mph x gear ratio x 336 / tire diameter
    so, if you turn 2,889 rpm at 65 mph with your stock setup and (just for the example) your stock tire is a 31in tire. you get
    2889 = 65 x 4.10 x 336 / 31
    If you want to return your truck to stock performance (before you added the bigger tires) you would just change the tire size and solve for the ratio.
    2889= 65 x Ratio x 336 / 38
    You would get
    2889= 574.736 Ratio

    Since 5.03 is not a real ratio you would move up to 5.13 ratio
    These calculation is for a trans that has a 1:1 ratio (usually 4th or 5th gear) With Over drive your rpms will be slightly lower on the freeway. Personally I would move up to a 5.29 or 5.38. It just depends on what you plan on using the truck for. Mostly highway? 5.29. offroad? 5.38. It is just my opinion. I went from a 3.42 to a 4.56 with 38.5's. Sure, I still got pretty good mpg on the freeway. But, I wish I would have gone to a 4.88 or 5.13. That being said, my truck was only a daily driver for about 6 months after I put the 38's on.

    Here is a chart that can be used as a quick reference, but the calculation should be performed to make sure.
    Green=Best MPG
    Orange= Best overall
    Red=Best power

  3. Mud_Slut7

    Mud_Slut7 New Member

    Thanks for the reply. I know the body lift would be a good way to go to avoid trimming, although the 2500HDs show a lot of frame as is so I'm not trying to show even more frame than already is.
    Thanks for the formula too, that'll help.

    Why'd you say it was only a daily for six months? something go wrong?
  4. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Nothing went wrong. I was driving over 150 miles per day and filling up the tank every other day. GM has some good lease deals on sonoma in 1998. So, I leased an extended cab 2wd sonoma for like $140 a month. I saved money doing so. Then when the lease expired I bought my 02 silverado ext cab 2wd. So, the big truck was only driven on special occasions and to off road.

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