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poor gas mileage!

This is a discussion on poor gas mileage! within the GM Trucks forums, part of the Vehicle Specific category; Another thing that hasn't been mentioned here is the season we're in. My rcsb regularly gets 17-18ish in mixed driving, ...

  1. #21
    Senior Member 5.0THIS's Avatar
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    95 Z28

    Another thing that hasn't been mentioned here is the season we're in. My rcsb regularly gets 17-18ish in mixed driving, and it's mechanically healthy (has the heavy 20 inch factory wheels on it). My current tank I'm at about 13-13.5 with similar driving. When it's this cold and it takes the truck forever to warm up, it spends a lot more time running pig rich. You're also fighting fluids which are much thicker than usual with the cold, and that takes horsepower. You're also running the defroster, and maybe letting it warm up more before driving. I know of plenty of these trucks with 100k+ miles with factory o2s getting as-new fuel economy.

  2. #22
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    navy blue
    2002 camaro ss

    To the above post yes thats a great point about it being winter time. Here where i live they use an ethenol blend for the gas in winter time which also hurts fuel mileage

  3. #23
    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by 5.0THIS View Post
    Another thing that hasn't been mentioned here is the season we're in. My rcsb regularly gets 17-18ish in mixed driving, and it's mechanically healthy (has the heavy 20 inch factory wheels on it). My current tank I'm at about 13-13.5 with similar driving. When it's this cold and it takes the truck forever to warm up, it spends a lot more time running pig rich. You're also fighting fluids which are much thicker than usual with the cold, and that takes horsepower. You're also running the defroster, and maybe letting it warm up more before driving. I know of plenty of these trucks with 100k+ miles with factory o2s getting as-new fuel economy.
    Yep, my duramax has dropped to the mid 15's lately city driving and I attribute that to the cool temps we've been having the last couple months (if you call 50's for highs cool) but it is pretty chilly in the mornings. In the summer time the truck was getting mid 16's to 17 mpg, all city driving.

    With the SD tunes I run, my wifes SS camaro for example, isn't affected by the cool temps at all. She consistently gets 21-22 mpg city driving year round. I believe that's because I run open loop and command the AFR I want with this tune, so the car isn't sensitive to changes.

  4. #24
    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by slimss View Post
    I have the same truck 99 silverado ext. cab LT auto 5.3 4 wheel drive. I get 15 mpg in town. It has 133000 miles. I put duals and a c a i. I had a 1996 ext cab chevy with the 350 auto and that thing sucked the fuel. id be lucky to get 12 mpg plus the turning radius was huge . I was glad to sell it.
    Wow, that doesn't sound too good on the 96. I assume that was a throttle body. My father has a 91 TBI short bed 4x4 with 33's and 3.42 rear, and the 700R4, he bought it new. Darn thing is getting close to 300k miles now, still knocks down 17 mpg with it. His newer 2002 3/4 ton 4x4 with the 6.0 and 4.10's with 33" tires (also bought new) and he is reporting 14 mpg driving the same roads. Both trucks are bone stock other than tires and a speedo correction.

  5. #25
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    Just put the sensors in today. Along with a new fuel filter.filter actually had a little restriction after only a year. Iwas kinda suprised. Anyway, Let you guys know what happens. Gonna throw new ac Delco platinum plugs in it pretty soon also. Along with an oil change. I've always ran synthetic in it thinkin that would help too. Not sure if it does

  6. #26
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    navy blue
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    Put the sensors in and have ran a full tank of gas and got an extra 100 miles out of the tank. Now I'm at almost 16mpg. I'm pretty happy that.

  7. #27
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    navy blue
    1998 camaro ss

    Oh and I gapped the plugs at 45 instead of 60 the last ones I had at 60. Calls for 60 but wasn't there a gm bulletin or something like that?

  8. #28
    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by Supersport0126 View Post
    Put the sensors in and have ran a full tank of gas and got an extra 100 miles out of the tank. Now I'm at almost 16mpg. I'm pretty happy that.
    That's pretty respectable for a truck and just about where it should be. It's amazing what some 02 sensors will do. These things get lazy over time and don't trim fuel like they should, even if they aren't setting codes. They are maintanance items just like spark plugs and everything else. Just one more thing to upkeep on a fuel injected engine

  9. #29
    Moderator Cutlass's Avatar
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    Quote Originally Posted by Supersport0126 View Post
    Calls for 60 but wasn't there a gm bulletin or something like that?
    Yes. GM/ ACDelco came out with an iridium plug that comes pre-gapped at .040" and GM released a bulletin to highlight that change. They are pre-gapped and shouldn't be adjusted because they don't want you to damage the tip or the coating. But IDK, I don't think regapping them would be a big problem.
    The old platinum plugs still carry the .060" gap recommendation.
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  10. #30
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    Damnit I put new platinum plugs in again and gapped them at 40. Should I take em back out and gap at 60? It's running fine. Might make some noise when cold but power is good. Or go buy some iridiums?

  11. #31
    Moderator Cutlass's Avatar
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    Nah. You're probably ok as is. I don't think you'll notice any difference between platinum or iridium...
    or .040 and .060.

  12. #32
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    Cool thanks man. Didn't feel like pulling them back out anyways. Even though they are nice and easy.

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