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  1. #1
    Jr. Apprentice
    Join Date
    Jan 2013
    Sparks, NV

    Default Need mileage increase; will changing gears help?

    Hi, I'm new to any thing like this, but I thought I might get some info/help from you folks out there. I have a '05 2500 HD regular cab truck. My question is I have the 6 Liter gas engine with 4/10 4/11 gears. My mileage is horrible and want to know if changing the gears to 3/73's would help the fuel economy any. I realize it won't pay for it self any time soon if ever but passing gas station occasionally would be nice

  2. #2

    mfleetwood's Avatar
    Join Date
    Jul 2010
    Denver, CO
    Blog Entries


    I moved your thread to the correct section and changed the title so that it relates to your content. This will get you faster and more responses than the Members Introduction section.

    It appears that you are not sure what gears you have. Did they come from the factory that way? What is the diameter of your tire, and any other relevant information you have will be helpful. Also, what do you consider "horrible" mileage? I don't think anyone who has a 6.0 gets great mileage...heck, I don't even get great mileage with a 5.3, but relatively speaking I am very happy with the MPGs I get.

    In any case, check out the gear ratio chart below, as this will give you a good idea what changing gears will do for you, but you will need to know your tire size. Also, I'd suggest checking your RPO code to make sure you know what gears you have now.

    Mike (Denver, CO) - 2008 Sierra 1500 Z71 SLE 5.3L 4WD

    SUSPENSION: Rancho 4" Suspension Lift; Rancho RS9000XL Shocks; Rancho Skid Plates; Rancho MyRide Wireless Shock Controller;
    BFG All Terrain KO 315/70/17
    DRIVE TRAIN: 4.56 Gears; Detroit TrueTrac Differential; True Cool 40k Transmission Cooler
    PERFORMANCE: DiableSport Predator Tuner; Custom Tuned by Diablew; Magnaflow Exhaust; AFE Cold Air Intake
    ELECTRONICS: Kenwood DNX6180 Touchscreen; Subthump box w/10" Kicker; Driver Information Center (DIC); Rear View Camera
    ACCESSORIES: Westin Brush/Grill Gaurd; Westin Nerf Bars; Truxedo Tonneau; 20% Tint; Tow Mirrors w/Heat & Signals

  3. #3


    First, welcome to GMTC!

    As for your inquiry, the delta between 4.10's and 3.73's is minor enough that it if I were in your shoes I wouldn't do it, as you'd give up a significant amount of low-end grunt for not much in the way of a fuel mileage increase ... and you're immediately out of pocket some cash. (This is especially true if your vehicle is 4WD since you have to do both differentials -- which costs a fair bit more since the front diff is labor-intensive.)

    Basically you'd need to step up to even longer gears to see significant mpg benefit on the highway ... and at that point you've rather defeated the purpose of having a 2500HD as you'll have neutered it of precious torque.

    Presumably you already watch your driving habits in order to maximize fuel econ. If that's the case, the next logical step would be to get a handheld programmer (such as a DiabloSport Intune) and work with a customer tuner (such as Diablew aka @06MonteSS on this site; you can PM him if you like) to develop a custom tune for your truck. There's usually some mpg benefit from doing so ... and it -can- be significant.

    Also, if you were to go through with a re-gear you'd need a programmer to tell your truck about the replacement gears ... or you'd have to make a trip to the stealership and pay them to tell your truck about the new gears. Thus, starting with a programmer and a custom tune is the best logical approach ... since the programmer does a LOT more than just tweak your computer one time ... and is always at your fingertips. (I always advocate that people go that route rather than spend 85 bucks at a the stealership for a simple computer tweak, as it's a better use of precious cash.)

    I believe 06monteSS bundles the programmer and a customer tune at a price that's cheaper than if you bought them separately, by the way...

  4. #4
    Jr. Apprentice
    Join Date
    Jan 2013
    Sparks, NV


    Thanks for the quick reply, mfleetwood, and SurrrealOne. It is 4x4 with 265 75 16 tires. 4:10 gears from the factory. I bought the truck with low miles needing a little bigger truck to haul my tools (I'm a carpenter), and materials from a '97 1/2 ton 4x4 chevy. Most of my driving is hi-way. With cruise control on at 68-70 I get 14.5 -15 mpg. The only changes I have made are Afe air filter, and after market muffler (? make). I use synthetic oil for life of motor. At 65 I'm turning 2250 rpm's (approx.). I think I like the sounds of the programmer and will look into it. Again Thanks.

  5. #5


    Quote Originally Posted by Ru-one View Post
    I bought the truck with low miles needing a little bigger truck to haul my tools (I'm a carpenter), and materials from a '97 1/2 ton 4x4 chevy. Most of my driving is hi-way. With cruise control on at 68-70 I get 14.5 -15 mpg.
    What did you need "bigger" for your tools exactly? Was the 97 1500 a short bed and you needed a full size bed? Do you need more hauling capability? More passenger space?

    If you don't need to do a lot of hauling, you might want to look at trading it back in, for a truck with a 5.3 AFM engine in it. You'll get much better MPG on the highway, particularly if there isn't much uphill driving. If you do need to haul a lot though, the 6.0 with the 4.10 gears will be much better, but your mileage isn't going to be great no matter what you do really.

  6. #6
    Jr. Apprentice wackoaboutgm's Avatar
    Join Date
    Jul 2012
    Belgrade, Montana


    Ru-one, your truck is almost exactly the same as mine- '03 2500 HD, synthetic oil, K&N air filter, same size tires, 4:10s and 12mpg town-15 highway ( don't have a heavy foot). I also have a steel headache rack plus a tool box with chains, tools of all kinds. Your truck is right in the ball park for fuel mileage.
    2003 SILVERADO 2500 HD
    2001 SILVERADO 1500 Z71

  7. #7
    Sr. Mechanic
    Join Date
    Jun 2011
    Tuscaloosa, AL


    hey man I've got the same truck for the most part but I do have the 3.73 gears and I'd keep what you have. Before I did anything to mine I was getting about the same mpg's as you. I made a few small performance mods including a custom tune and after it's all said and done I probably gain 2-2.5 mpg.
    2004 GMC Sierra SLT 6.0L 4x4
    UWS Low Profile toolbox 10% tint front and over stock back tint
    AirAid MIT and K&N airfilter
    BlackBear Performance

  8. #8


    Welcome to the club, it's great to have you here.

    2002 Chevrolet Silverado Extended Cab SportSide: See Mods in My Garage

  9. #9


    Quote Originally Posted by Ru-one View Post
    I think I like the sounds of the programmer and will look into it. Again Thanks.
    This will be your biggest bang for the buck. You might find 3-10% MPG improvement with a custom tune. I know my 6.0 rig (1500) picked up 1.5MPG with the custom tune after just your improvement (better air filter and muffler). At $250-$350 you'll definitely pay it off over time. Add that to the fact you'll end up with more power and a few other enhancements of your choice (like better shifting), and it's probably the single best improvement you can make, especially for the price.

    Cheers, and welcome to the club!

    2006 Vortec Max 1500
    -Custom Tune (389HP 440 Ft/lbs Trq - Dyno'd) | Volant CAI | Magnaflow Dual-in/out Exhaust w/3" stainless pipe tips | Fully Built Transmission w/ Red Eagle Clutches & Kolene Steels w/ Corvette Servos and Stage2 Shift Kit | 35K Tranny Cooler | Mobil 1 | Royal Purple Rear Diff
    -Spray-in Bed Liner
    -Premium Sound w/lifetime Satellite Radio | Leather | Sunroof | Heated Seats
    -Limbstriping from USING the truck (those are badges of honor)

  10. #10
    Jr. Apprentice
    Join Date
    Jan 2013
    Sparks, NV


    Thanks for all the input, Concenses seams to be a programmer. I'll be looking into that, trading in right now is not an option, trucks don't get the best mpg any way some improvement is better than none. Happy and safe driving to all.

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