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Numerous issues with my truck. Need a little assistance

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by texastito, Jun 25, 2012.

  1. texastito

    texastito New Member

    How's it going everybody? I'm definitely not a new chevy 4x4 owner, but my latest purchase has definitely rubbed me wrong, and has me wanting to strangle the previous owner. not to mention simply trying to figure out how to get better performance and MPG's (this is the absolute worst I've ever had).

    So, here's my specs:

    07 Chevy 1500 Crew cab; Z71; LTZ, 33x12.50x20 tires

    Here's several of my issues:

    -I get and avg of 13.5 MPG (city/hwy). Hwy I get 13-14 MPG. City I get 13-14 MPG. yes, that's correct, no change from city or hwy driving.

    -The power and performance in this truck really sucks. My previous '04 GMC ext cab 5.3L with oversized tires (dont recall exact size) wasn't a hot rod by any means, but you could feel some power. This truck - none whatsoever. Hwy passing speeds - when you floor it, it drops to lower gear, but barely seems to accelerate.



    The previous owner capped off one side of the dual exhaust muffler outlet (single in, dual out). I know this contributes a little to my issues, but how much? Would this be a major cause of my problems?


    Also, just some other info - at 70mph, my rpm's are a little over 2K. This seems quite high, as every previous truck Ive had ran several hundred RPM's below 2K.

    Any tips?
  2. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    I'm not an exhaust guy so I can't speak to your capped muffler issue. I'll wait for someone who knows far more than I do about exhaust to answer that ... and hopefully learn something, myself.

    As for your performance, the wheels and tires on your truck have been upsized from whatever the factory size was ... to what you have now. If the front and rear gears of your 4x4 were not also changed (to preserve the original performance of the truck) then you should absolutely expect it to be a dog. You should first determine your factory gear ratio ... using the RPO codes found in your glove box. As you're not a new Chevy owner I suspect you already know how to look up your RPO codes, so I won't detail it, here. Next determine your OEM tire size ... specifically the overall diameter. If you don't know then check the spare if it's still on the truck (and remove it, if so, as you're carrying dead, unusable weight around which hurts your MPG) ... the spare's overall diameter should be matched to that of your factory tires. You can plug its specs into http://www.miata.net/garage/tirecalc.html to get the overall diameter in inches. You can also use that site to compute your speedo error from having upsized your tires. :)

    Now that you know the original tire size and gear ratio, plug them into this chart (which presumes 65mph on a 4-speed tranny that is NOT in overdrive; hence the high RPM's in the chart) ... and that'll tell you roughly how the truck was set up. Next, plug in your CURRENT tire size and gear ratio ... and you'll see about where you fall in relation to the factory setup ... and can look across the chart to see about what ratio you'd need to get back to the factory setup. (You can also tweak it to a longer gear if you want more MPG at the expense of performance ... or a shorter gear if you want more power at the expense of MPG.)

    You can solve a big chunk of your performance problem by regearing both your front and rear gears (you have a 4x4 so you MUST do both if you do it, at all) to a ratio that suits your preference for power or MPG. The chart work gives you a rough idea of where to begin. (There are more detailed gear ratio calculators out there ... the charts will always be rough approximations.) Analyze how much of which (city/hwy) type of driving you do, as well, since it'll play into the gears you choose, too. Lots of highway means longer gears ... for MPG, but not so long that you fall out of overdrive on mild inclines. You lose top end with shorter, more powerful gears; my 4.56's are great for getting me moving but I seldom drive more than 68-70mph on the highway because my MPG falls off quickly beyond 65ish with those guys...


    GearRatioChart1.jpg


    Next is MPG. I saved it for last because your performance problem plays into your MPG issue. If your gears are too long then you lack the power/torque to get those big meaty tires and chunky 20" wheels moving, so you're going to step on the gas more and for longer in order to get rolling from a stop or as you come out of a curve... which hurts MPG around town. If your gears are too long then you also lack the power to stay at a given speed on mild inclines and you'll drop out of overdrive (i.e. downshift) a gear (or worse, two) to maintain speed ... which hurts your MPG at highway speeds. Keep this in mind, as it's unintuitive for most people that a shorter, higher power set of 4.56's (or 4.10's, or whatever) can get MPG's back ... but they can .... depending on tires, wheels, and your driving habits.

    That's relevant because a tuner should help you. You can run a canned tune for 87 octane that'll get you a little bit of MPG back. To get more you should consider a tuner that lets you work with someone who builds custom tunes ... as every truck is unique. @06MonteSS (aka Diablew) on this site bundles Diablosport tuners with his custom tunes in terms of pricing; PM him with questions. I've worked with him and recovered 2.5MPG from tuning, alone. Granted, it was MPG I lost due to bigger tires and heavier wheels, but it's recovered, nonetheless. I can't guarantee you'll get those results, as every truck is different, but I can state that you'll get more from a tune customized for your vehicle than a canned tune made to work well with any vehicle. You can also correct your speedo error with a handheld tuner (something the stealership charges a lot for) ... and if you regear you'll need a tuner to adjust for your new gears (again the stealership charges a lot for this). You'll gain some performance from a custom tune, as well, since Diablew touches the shift points, usually, when he customizes a tune.

    Between those things and the fuel savings, a tuner pays for itself relatively quickly ... as does a custom tune ... especially as fuel prices go up. Mine both paid for themselves in less than 6 months. My regearing got me another ~0.5 to 1 MPG back in addition to substantial performance improvements, but at 3x the price of the tuner and custom tune it's taking longer to get that cash back in savings, as the MPG gains were smaller and the cost was much larger.

    Last, check your driving habits. From your mpg's it sounds like you drive fairly mildly. Still, it's worth a look...
    Last edited: Jun 25, 2012
  3. 06MonteSS

    06MonteSS Rockstar 4 Years 100 Posts

    nice! good info there man!! ^ ^ ^

    ;)
  4. LOVINTHESTORM

    LOVINTHESTORM Active Member 2 Years ROTM Winner 1000 Posts

    Surreal you are the man! Great info spot on.
  5. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    Being a fellow craftsman, I hate to take exception with a man that can put together the info that SurrealOne has posted :lol: ; but I would like to add:

    - The 20" wheels are probably stock, the LTZ trim package includes 20" wheels; however, the tires are bigger than stock

    - The spare will be a 17" steel wheel, again, factory stock.

    - The factory tires have been selected, so that the 17", the 18" (Z71) and the 20" wheels will all have the same rolling circumference.

    - If the original design was for twin tailpipes, the engine is probably the 6.2 litter, not the 5.3.

    - blocking one side, of muffler output, would probably effect both mileage and power.

    Your mileage may be bad for a number of reasons,: is the air filter clean; if a 6.2, are you using high octane gas; do plugs need to be changed; if plugs need to be changed, change the O2 sensors as well.

    RPMs at 70 should be below 2k, or at least around 2k; unless, as SurrealOne suggests, the gears have been changed.
  6. TimTom64b

    TimTom64b Member 2 Years 500 Posts ROTM Winner Gold Member

    I am just going to add my opinion to the capped off exit port on the muffler. The muffler was designed with two exits... capping one side is suverely impacting function. Usually the dual exit mufflers have smaller exit pipe sizes compared to single exit mufflers. My guess is if you fix this you will decrease backpressure and increase efficiency of the engine... and that 5.3 will come back to life. This is the first thing I would do
  7. LOVINTHESTORM

    LOVINTHESTORM Active Member 2 Years ROTM Winner 1000 Posts

    I definitely agree with TimTom64b.
  8. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    No need to hate taking exception to it; you clearly know more about the LTZ trim packages than I do (I thought they came with 18" wheels), so by all means make corrections if they are appropriate. I've got no ego in this to bruise. I appreciate you adding to the data, here, as that'll help [MENTION=55365]texastito[/MENTION] the most!
  9. RallySTX

    RallySTX Member 100 Posts

    Welcome to the forum, open up that exhaust, check what was mentioned earlier, and add your general location to your sig so we have an idea what your terain looks like. For instance, if I had just said WI for my location, folks would assume, ok rolling hills and plains, but I said Janesville because here we have no less than five main routes through town that have a 20% grade on them. There should be a VECI label under the hood with the engine displacement in litres listed. I would just cut that whole fake dual exhaust off the truck, and just run one big pipe out the back. You might save a few pounds in weight, get increased power, and save money on future exhaust work. I never could understand 2 into 1 into 2 systems.
    Brian L.
  10. Bigmacnchez

    Bigmacnchez Member

    I third the thought !

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