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C5 poor mpg

This is a discussion on C5 poor mpg within the Corvette forums, part of the Vehicle Specific category; I just bought 1998 C5 installed *new plugs *new wires *new fuel filter & pcv *new battery *Dynotune gained 47hp ...

  1. #1
    Warren
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    C5 poor mpg

    I just bought 1998 C5 installed
    *new plugs
    *new wires
    *new fuel filter & pcv
    *new battery
    *Dynotune gained 47hp went from270rwhp to 317rwhp
    problem is now the most I can get is 21mpg Highway
    most of my driving is distance. Is there anything I can do?

  2. #2
    Ahh crap... needmorepwr's Avatar
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    '57,'63,'70,'91,'01

    did it get better mileage before the tune? I assume its an automatic?

  3. #3
    Warren
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    It got better mileage before the tune and its a six speed it got 27mpg before the tune

  4. #4
    Senior Member ss~zoso~ss's Avatar
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    SS: NBM, tan top
    1998-SS, 2010 Jetta TDI

    hmmm, thats a stumper

  5. #5
    Senior Member MadSeason's Avatar
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    1994 POS

    Any way to check if you're running rich?

  6. #6
    Warren
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    It was running rich, but the tune took care of that. Im new to vettes, how many miles would I have to travel in orderfor the computer to register higher mpg. 50 100 150 200.I ride 66miles to work at 60-70mph still get 21 at best

  7. #7
    Chad711
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    21 at best is great IMO. I can never get over 16 mpg in my car. But I don't use 5th or 6th all that much either, lol.

    I also have a pretty rich tune on my car. I think 21 mpg is pretty good man. You are not really to far from avg any way.

  8. #8
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    Quick Silver
    2001 Corvette Z06

    I wonder about that.... I know my Z06 is a little different, but I get around 30 mpg at 70 mph on trips... I am currently traveling about 80 miles a day round trip between home and work locations and still get about 375 miles per tank between city and hwy usage.
    Don't know what it would be, but hopefully the pcm just has to readjust???

    Danny
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  9. #9
    Warren
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    Im going to clean my air filter maybe that will help gain that elusive 28mpg

  10. #10
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    MY
    2003 Corvette Z06

    I got 27 best before my H/C headers install. After the tune I'm now getting 31 Hwy if I'm trying. Go figure...

  11. #11
    jdec
    Guest
    That's a strange one. I drive my vette everyday and most of the driving is in town. I get anywhere from 20 to 23 in town and still pull down 28 to 30 on the hill and curve filled ozark roads. Good luck figuring this one out.

  12. #12
    Bombguy99Z28
    Guest
    Who tuned it?

    Most 'dyno tunes' tune for WOT only, and sometimes the adjustments they make to get your car where they want it at WOT end up compromising part thottle.

    Do you have a unported, screened MAF? Sometimes running a ported MAF will bump your fuels trims to the moon.

    Have you checked your tire pressures lately? Low tire pressure can lead to substantially lower fuel economy.

    Do you have access to any data logging software?

    It should definately be higher than that, I get 19-21 driving 60/40 city/highway (with a bit of 'playtime' mixed in), about 29-31 on straight highway driving, all based off tank fillups and not the dash. My speedometer is 100% spot on. My dash gauge for some reason reads about 1mpg high.
    Last edited by Bombguy99Z28; 12-26-2005 at 10:07 PM.

  13. #13
    Warren
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    Mpg

    The Vette Doctors dynotuned the car. I gained almost 50 rwhp
    from270 to317 The fellow I bought the car from installed a recalibrated mat
    sensor. Last night from NY to Middletown aprox 70 miles aprox 21.4 mpg @65mph. She does have 124,000 on her.

  14. #14
    Bombguy99Z28
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    I'd toss a scanner on it. I get the above mileage with 90k on the clock.

  15. #15
    Junior Member Corvettestripes's Avatar
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    Pewter
    Corvette C5

    ttt

  16. #16
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    pewter
    2000 lingenfelter 383

    your mpg should not drop that much unless your always 100% on the gas... with a correct tune your car becomes more efficient than stock.

  17. #17
    Junior Member crawlingt's Avatar
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    black
    92 Sonoma GT

    maybe disconnect your ground and let the comp reset itself?

    Just an idea

  18. #18
    Groucho
    Guest

    Hi neighbor...

    Quote Originally Posted by Warren
    The Vette Doctors dynotuned the car. I gained almost 50 rwhp
    from270 to317 The fellow I bought the car from installed a recalibrated mat
    sensor. Last night from NY to Middletown aprox 70 miles aprox 21.4 mpg @65mph. She does have 124,000 on her.

    Reset the D.I.C.
    I get 27 or 28 always in the summer...but 23 or so in the cold...

  19. #19
    tsts
    Guest

    Thumbs up Mpg

    I'm sure you got an excellent tune from the vette doctors, they have a very good reputation for their work. There is another approach you may want to consider, "street tuning" where your engine is tuned and checked for many variables and performance setting while actually being driven. I had Chuck at Corvettes of Westchester tune my '99 LS1, A4 and the difference was awsome. I would guess the Vette Doctors can do the same type of tuning and are good at it, there are soooo many things they can change for better street performance through all types of driving. Also, If you have a forced air system (I think its called varamm or something like that) the car isn't moving when its on the dyno so its not working for you at that time. (if you got a stock air filter - disregard the last comment, but make sure you air filter is clean). Good Luck.

  20. #20
    Senior Member ss~zoso~ss's Avatar
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    i averaged 32 MPG with my M6 camaro (only highway driving)
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