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  1. #1

    Default What's changed from 03 to 09 to kill performance and gas mileage (both 5.3L w/3.73)

    Had an 03 Silverado 1500 Extracab with 5.3L and 3.73 gears and it averaged about 17mpg highway with a lifetime average of 14.6 and that's towing sleds in winter and camper in the summer. It would smoke my newer truck both gas mileage and power. On trips I could squeak 18-20mpg if I stayed under 70mph.

    Picked up a new 09 GMC 1500 SLT Extracab that also has a 5.3L with 3.73 gears and it's a dog both performance and gas mileage wise. Old truck could smoke the tires in 1st/low just by stomping on the gas. I could also clear a 2mi 7% grade hill at 100mph if I wanted to. New truck couldn't spin a tire if I wanted to, gets about 13-15mpg on the highway and can only do 86mph up that same hill. The 09 has 18" wheels where my old truck had 16.5". Even cruising 60mph on the interstate I can only get up to about 17mph and if I drive my usual 72-73 I get about 12.8-13.2 mpg.

    I talked to one performance shop and they said it's likely a different, (less efficient transmission) and the drive by wire. I could see the drive by wire taking the WOT away at the start but it should do WFO once rolling I'd assume. Mentioned it to dealer and they said unless it triggers a code there is nothing they can do.

    My buddy has an 08 Crewcab and we set the cruise at 70mph on this long flat, his was getting about 23mph and was in 4cyl mode, my truck was in V8 mode and getting about 15. When we race he blows me away with any side by side comparison and I assume his truck is heavier being a crew cab. He also has 5.3L with 3.73 gears.

    Buddy has a new truck with the 6.2 and got 18.9 on a long trip walks all over my truck both in power and gas mileage. Is there anything I should do to or is it the nature of the beast and there are just good ones and bad ones.

  2. #2


    betcha I could make your 5.3 eat your buddy's 6.2... ;)

    unless I've already tuned his, then all bets are off

    2014 Camaro SS - 2SS, RS pkg., short-throw shifter, NPP perf. exhaust, BMR strut-tower brace, Cold Air Inductions cold air intake

  3. #3


    It sounds like the issue lies with your truck, though I can't start to begin to wonder where absent driving habits. Your buddy with the 08 is more typical of what I'd expect to see. I have an 09 crew cab and see numbers more closely resembling your buddy's than yours. My opinion would be to not ask what changed from 03-09 and instead look at what's wrong with your truck.

    To Be Completed (Eventually):
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  4. #4


    something is wrong with yours somewhere. my 11 is doing a 16 mpg average (mostly city) and has good power. i went form a 05 with the 5.3 extended cab with 3.42s to a 11 crew cab with a 5.3 with 3.42s.

    the 11 is just a quick. mileage is very close.


    2011 GMC Sierra SLE 5.3 Z71 4X4 Stealth Gray Metallic / 2004 Chevy Impala LS 3.8 Cappuccino

    Tow mirrors - Diablew Tuned - Flowmaster Regular 40 - Ready Lift 2.5' lift - BFG LT A/T K/Os - Carr Light Wing - TruckLite LED lights - Optima Red Top - 50% Front Window Tint - Line-X bedliner - Airaid MIT - Tekonsha P2 - ARS Billet Grill - Fia custom fit seat covers

  5. #5


    Is the truck stock or modded? I ask because if you're lifted with big (35") tires under it I'd expect it to be sluggish with 3.73's ... and also be unable to spin those tires.

  6. #6


    My 2010 is less motivated than my 2007 and my 2007 was a 4.8 with 3.42 gear. In cruise mode the 07 took the Pequops much better than the '10
    2010 Chevy Silverado Z71 ext cab.4x4
    72 Monte Carlo 350-415 hp- sold (June 2011) to pay medical bills
    79 Impala 350 4bbl 375 hp- sold (June 2011)to pay medical bills

    "If He asks you to go a mile go two"

  7. #7


    Quote Originally Posted by SurrealOne View Post
    Is the truck stock or modded? I ask because if you're lifted with big (35") tires under it I'd expect it to be sluggish with 3.73's ... and also be unable to spin those tires.
    Both trucks stock. A couple months ago I added a 1" Pro-Comp leveling kit and went one tire size up from a 265/65/18 to a 275/65/18. It was a dog before the change and all comparisons were done back with the original tires/setup. I haven't noticed any real performance change but I did see a drop in gas mileage by about 1-2 mpg.

    My old truck was suppose to be 285hp and I think the 09 is rated for 315 or 320hp but I'm not positive on that.

    The drive-by-wire is probably what's taking away the snap at lower speeds under 30 but I still don't get what kills the performance compared to other similar trucks on hills and highway speeds. I don't know if my buddies truck has the 5/6 speed tranny or not, mine is the classic 4speed which I think is different than the 4 speed that was in my 03. It's geared great for passing (40mph-70mph) but if you're going much over 70mph up a hill on the interstate it won't shift down even if I pull it down and it just drowns along up the hill. If I drop to low 70's it will shift down, then stay in 3rd up to 85/86mph but once it shifts it's done and can't pull another mile per hour.

    Again both trucks have 3.73 gears but if you didn't read it on the spec sheet my old truck acted like 4:10 gears where the new one acts like 3.42's.

    If I knew I could get some major low-cost gains by chipping it and cold-air intake I'd do it but I don't want to go crazy with mods as I'm just holding out until the new model comes out then will trade. Truck has had a shimmy since new and even with the new tires it's still wallows. The 03 had the Z71 and bumps in the road were solid, the 09 has the towing Z85 package, (not sure if stiffer or not than Z71) and it shutters like my old 66 olds convertible and like it's under-damped or something.

    Oh well, it is what it is. The old truck had way more features not the trip meters such as lifetime avg, personal/truck trip meters, etc, seems like they went backwards on the 09 options.

  8. #8


    Something seems odd. If you're going to trade it then it sounds like there's nothing to do but wait. I wouldn't expect to see gains other than at WOT for a CAI so that's out.

    A handheld will get you some MPG back but you have to do a custom tune to really get the most out of it. I got a ~2.5mpg improvement out of my handheld and custom tune, but that's no guarantee that other trucks will get the same; still, even 1-1.5mpg is enough that one might might consider a handheld programmer and custom tune as it'll pay for itself in fuel savings. And while the custom tune won't migrate to the new truck ... you can certainly take the handheld to it.

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