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Thread: Speedometer accuracy

  1. #1
    Advanced Tuner
    Join Date
    Oct 2004
    Location
    Orlando FL
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    766

    Speedometer accuracy

    I borrowed my neighbors TomTom 720 for a trip to Sebring Raceway, about 140 miles one way. One the way down, I noticed that my speedo was a good 4-5MPH higher than the GPS at pretty constant cruising speeds. After reading that manufacturers often make it look like you're going faster to reduce tickets (and maybe make you think your fuel economy is better than it really is?), I expected it to be closer with a a better calculation of the tire height.

    On the way back, I stopped once to add 6% to the revolutions, then added another .5% because sometimes it would still be 1MPH low, and I'll add another .5% soon because it still is a bit low. So once that was close, then I filled up the tank until the handle stopped automatically and drove home 69.5 miles, where I stopped to get gas. I clicked the handle to add gas, and it stopped after ~1.5g. That can't be right, so repeat and it stops at ~1.9g. That can't be right either, so I just pull the handle slightly to let it flow slow, and it stops at 2.125. So my MPG was ~31-32MPG with the final fill.

    That seems too high, did I do something wrong? I can post the calibration and log if desired. No quasi lean cruise tricks in this tune, and my cruise control button doesn't work so it was manual cruise control if you will.
    Last edited by JimMueller; 11-09-2008 at 09:13 PM.
    1998 NBM Camaro Z28
    LS3 motor
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  2. #2
    Advanced Tuner Programmer's Avatar
    Join Date
    Jul 2007
    Posts
    244
    I've seen as high as 40mpg with a nice tailwind in my '99 vette (Cam, heads, headers, FAST, TB, etc....)....so I'd think it's probably right Jim !