How "off" is your odometer

Discussion in '2012-2015 VW Passat TDI forum' started by amsoilguy, Jun 5, 2012.

  1. amsoilguy

    amsoilguy New Member

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    Hi all. The odometer is off by + 1.0 miles, over a 30 mile stretch. Went 31 miles even though the odometer indicated 30 miles. I checked this using the mile markers on the highway. Filled up today and hand calculated according to the shown odometer reading I got 49.2 mpg.(17.137 gallons used) The MFD indicated 51.0. When taking into acount the mileage discrepancy, the with 843.2 miles traveled, would indicate an additional 28.1 miles, for a total of 871.3 miles giving a hand calculated 50.84mpg. How different is you odometer from actual? I have the 18 inch tires on mine, not sure if that makes that much a difference, unless the circumference difference between 17 and 18 is that significant.

    At 1/16 on the fuel gauge, still had 1.3 gallons in the tank. That is assuming that the 18.5 gallons is accurate. They really had done work on this guage as it is damned accurate from full to empty....
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  2. gfr1

    gfr1 New Member

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    You are on questionable ground here. I'm not aware of posted sureyed five or ten mile calibration locations as there once were. Interstates are surveyed, yes, but road change errors are factored in and are not apparent. When you are at a mile marker, that is the same location along the road on both sides, which ever direction you are traveling. Along a large bend and around hills, you might tavel a different distance traveling the opposite way, but will begin and end up at the same mile markers. They don't re-post when a road change is completed, either, other than the local area. If they did, they would have to re-survey across the remainder of the whole state and move all markers. This type calibration could be a bigger error factor than reading your instruments. Then our figures will end up worthless. Then, with GPS, the ommitted tenths, or whatever the measurement is, are dropped. Another words, 19.99 miles or 19.0 miles will both display "19" (at least on most standard automotive/portable systems). Also, the GPS reads distance and speeds over the surface of the earth. An odometer reads the road surface. In theory, if you had a theoretical triangle on edge and you traveled 10 miles up one side and ten miles down the other, the odometer would read 20 miles, but the GPS would only read ten (across the third side), which would also be reflected in the speed. In a high performance fighter jet, with inertial nav or GPS, when you pull straight up, traveling a few hundred miles per hour, the airspeed shows your velociy through the air, but the inertial nav and GPS systems show "0" movement over the ground, assuming that you remain over a point during the maneuver. If you could actually go vertical up and down, the inertial navigtion and GPS would display the wind vector at the altitude of the reading, as you are being blown downwind. (Been there, done that) Measured distancs are hard to find, as are "roller" calibation stations. AAA used to have some mobile units for this, but I haven't seen them in a few years. And, I always wondered about tire radius issues becaus the rollers caused a pretty good depression in the rolling radius, depending on the tire pressure. I have a $500.+ "Race Logic" GPS instrument for such things (as used by performance people), but haven't measured a distance with it as yet.
  3. amsoilguy

    amsoilguy New Member

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    Point taken. Will check mile markers in oposite direction to see how wrong i am. Daily commute, so doing it all the time.
  4. domano 68

    domano 68 Member

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    After my first tank my mpg gauge was showing a higher mpg than my hand calculation. So I decided to check my odometer. I used a gps app Map My Run. Over 25 miles the odom was off by -0 .4 miles on the morning route. It was off by -0.5 mile on the same 25 mile route this afternoon. (gps showed 25 miles, vw showed 24.6). So this discrepancy does not line up with my mpg gauge being higher than the hand calculated #. Hmmmmmm!

    I also noticed odom was showing lesser distance than mile marker posts.
    Last edited by a moderator: Jun 14, 2012
  5. amsoilguy

    amsoilguy New Member

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    Does or doesn't it?

    Domano,
    Does or does not? Seems to me that you negative distance 24.6 (VW) vs 25(Actual), fits perfectly. 25/2.5 is a greater number than 24.6/2.5.... So hand (VW Odom) would be less than MFD (GPS Actual) right?
  6. Knotty

    Knotty New Member

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    Over flat ground with cruise control on my speedo reads 65 while my GPS shows 67 MPH. Assuming GPS to be accurate, the speedo and presumably odo are 3% low.
  7. amsoilguy

    amsoilguy New Member

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    :+1
    That almost matches my 1 mile on 30 exactly. Keep forgetting to do the route the other way.... On the way home then and have better things to think about!!
  8. Yamaguy

    Yamaguy New Member

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    My new Passat Speedo shows just like yours when compared to my Garmin gps.
  9. damonator

    damonator New Member

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  10. gt4494

    gt4494 Member

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    The best answer I have found to this subject is that according to the UN standards
    http://www.unece.org/trans/main/wp29/wp29regs21-40.html Look up reference - UNECE 39/00 (Rev 1)

    all speedometers must not read less than the actual speed of the vehicle. To ensure compliance I understand VWs are designed with an approx 3 mph error to the good (high).

    Since speedometers and odometers are now electronic and not mechanically connected the odometer does not have the same error programmed into it.

    If anybody has any better answer please respond.
  11. shoremnj

    shoremnj New Member

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    Actually if I set the cruise at 70mph and use external gps to measure speed it shows 73mph
  12. amsoilguy

    amsoilguy New Member

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    OK. How does this explain my positive 1 mile on 30? 31 total. showing 30 on the odometer? I think I am going faster too, just as another poster indicated. I do not know how accurate those road side radar signs that PennDot puts out, but that reads over as well. I think that this is more than enough proof for me. 60 MPH in less than 60 seconds per miles says it too.
  13. lwt42

    lwt42 Well-Known Member

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    If the odometer reads 3% low, isn't that a 3% warranty extension?
  14. gt4494

    gt4494 Member

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    Southeast Coast on I-95. Radar detector on and yes Herbie still gets 40mpg at 80 mph
    Not sure how to explain your error but I know the mile markers along I-95 b(below DC) are anything but accurate. The speedometer error in all my VW's has been approx 3mph high reading (against a GPS).

    Never have been too concerned about the odometer reading.
  15. amsoilguy

    amsoilguy New Member

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    Well everything around and in DC is screwed up:laugh:..... Well at least the politicians!:annoyed
  16. amsoilguy

    amsoilguy New Member

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    Tire make a huge difference

    All,
    Go to the Tire Rack and compair the revolution per mile between our 18" hankook tires and others on the site of the same size. Revs per mile can be 30+ less different. So changing to another tire may skew the mileage recording as well as the miles per gallon fairly significantly, not to mention make your actual speed higher than shown. Does the TMPS system adjust the computer to change the spedo?
  17. TDISEL

    TDISEL New Member

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    GPS would eliminate any guessing.
  18. ColoradoDriver

    ColoradoDriver New Member

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    You can correct the odometer and digital speedometer on the 2012 Passat using VCDS. Here is how to do it:

    [Select]
    [17-Instruments]
    [Coding - 07]
    [Long Coding Helper]
    Click on each of the Byte numbers in row 3) and you'll see the selectable options below.
    In this case, Byte 0 contains the "Distance Impulse Number".
    2012 Passats are set at "3" by default. If you change that number to "2" it will improve accuracy when using stock wheels/tires.
    In my case, using 225/50R18 tires, I changed the value to "1" and got the results I wanted.
    That fixed both the odometer and digital speed display. The analog speedo is still a bit optimistic, but I usually use the digital speedo anyway.

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