My gas mileage just disappeared. Help!
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This is a discussion on My gas mileage just disappeared. Help! within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; My car is a 2004 STi. I just bought this car at 70,000 miles. All my driving is in town ...

  1. #1
    Registered User Mr Giggles's Avatar
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    Mar 2010
    Tulsa, OK

    My gas mileage just disappeared. Help!

    My car is a 2004 STi. I just bought this car at 70,000 miles. All my driving is in town and I don't drive the car hard or brake quickly. The last two tanks averaged ~13mpg. I was shocked, so I checked my tire pressure. The front wheels were 10psi under the reccomended 36 and the rear tires were at 19psi rather than the reccomended 30. I'm wondering if this could be the cause of the mileage drop or if it could be something else. I just changed the oil and got new spark plugs the day I purchased it, so those aren't the cause.

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  3. #2
    Registered User mudferret's Avatar
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    Aug 2009
    Tire pressure that low will definitely impact your mpg pretty hard. What mpg were you at before? The STI isn't exactly a hyper miler even if you do try to drive like a granny.
    MMVI WRX - VF43 Airboy Tuned @ 3500'

  4. #3
    Moderator T0rque's Avatar
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    Dec 2004
    Medford, Oregon
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    what plugs did you install? Did you change the fuel filter? How is your air filter?

    Lead Wrench @ WTF Tuning, LLC

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  6. #4
    Registered User Relyt's Avatar
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    Mar 2010
    Northern Kentucky
    Quote Originally Posted by LPPTundWRX View Post
    what plugs did you install? Did you change the fuel filter? How is your air filter?
    I'm going to blame it on someone stealing your car while you were sleeping and hammering on the peddle. Or someone syphoning your gas :-).

    In all seriousness give it another tank or two after you fill the tires and you will have your answer.

  7. #5
    Admiral Ackbar the 1st mycologist's Avatar
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    Oct 2005
    Late Devonian
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    Quote Originally Posted by lpptundwrx View Post
    what plugs did you install? Did you change the fuel filter? How is your air filter?
    "From a little spark may burst a mighty flame." - Dante
    "The stitch is lost unless the thread is knotted." - Italian proverb

  8. #6
    Moderator Donkey's Avatar
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    Apr 2005
    Da boonies,Va
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    Like mentioned above,tire pressure can greatly impact MPG's. So will state of tune. List your mods.
    Resident Adviser @ WTF Tuning, LLC
    2002 FORESTER S
    1998 FORESTER S

  9. #7
    "Supreme Subaphile Extraordinairre" ninefourteener's Avatar
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    Mar 2006
    SLC area
    I noticed something interesting about my car.

    As boost goes up..... so does fuel pressure..... regardless of the RPMs.

    You would "think" that by "grannying" the car around town, and shifting a low RPMs would save gas...... It doesn't.

    When you shift too soon, you're in a gear too high for the speed you're going... so the turbo wants to compensate by boosting, even under the slightest acceleration. If you're turbo is working... so is your fuel pump.

    It is actually BETTER for mileage to accelerate slowly...... but shift at a higher RPM..... keeping you at a constant state of vaccuum, not boost.

    Let's say you're driving up a mountain at 65 mph.

    In 5th gear.... you're probably boosting 3-5 psi, and pumping approximately 35-40 psi of fuel, even though you're engine is only turning maybe 3k rpms.

    In 4th gear..... you're likely not boosting at all, and only pumping 20-25 psi of fuel, but you're turning nearly 4k rpms.

    .... and you're getting better gas mileage in 4th.

    Long story short.... maybe you altered you're driving style to try to save gas.... and it ended up working against you? In a non-turbo car, it works. In a turbo car, it doesn't.
    Last edited by ninefourteener; 03-30-2010 at 09:23 AM.
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