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Odometer Error Constant At All Speeds

Discussion in 'Prius c Care, Maintenance and Troubleshooting' started by Mr Incredible, May 5, 2012.

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  1. Mr Incredible

    Mr Incredible Chance favors the prepared mind.

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    (I meant speedometer but can't change it.)

    Even to 100 indicated. Scangauge says 99.

    I found the error in my vehicle to be about 1.1 MPH fast with the 16" 195'S's. 38 psi cold, 92 degree day. Scangauge and speedometer were as closely in synch as I've seen them.
  2. F8L

    F8L Protecting Habitat & AG Lands

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    Interesting. Was the error constant at say 25mph? I did quite a bit of testing in my GenII and the 2mph slowly went away as I dropped from a higher speed down to 40mph or so. At higher speeds it was fairly constant although I never checked it at 100mph. I assume you reach full tire expansion by 50mph or so.

    In my GenIII the Scanguage is 2mph too fast the majority of time but at low highway speeds it cold drop to around 1.5mph too fast but I admit it is difficult to watch the speedo, scan gauge and the GPS on my phone. :)
  3. JamesBurke

    JamesBurke Active Member

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    All speedometers report high. It's a weights and measure liability issue. Just like all things you buy contain a little more by weight or volume then the label says. If they were short you would feel ripped off and they would have to recall all the product. If you were ticketed for speeding because your speedometer reported a low mph and you could prove it you could sue for damages, (fine, points, insurance penalty ect). The manufacturer would have to recall all those low reporting units.

    Good to know what the offset is anyway for the various OEM tires.
  4. Mr Incredible

    Mr Incredible Chance favors the prepared mind.

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    As far as I can tell the 1mph speedo error is constant from below 25 up to 100 (indicated).

    Whether or not it's a Toyota or U.N. conspiracy I can not tell.
  5. MNGoan

    MNGoan New Member

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    Probably the best calibration would be to a GPS system. Having said that, all the cars I have ever had read an mph or 2 too high.

    I have never been concerned about the speedometer, as much as say like the gas gauge. My Ford Contour has a fuel pump where the gas tank is empty, and the gauge is sending a quarter tank reading. Hopefully the Prius has a more reliable gauge?
  6. alfon

    alfon Active Member

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    Ours speedometer is off 1 mph as well,over reading, has been since new and it appears it is not self correcting.

    The MPG gauge is also off by 3 mpg over reading, so
    I am not surprised.

    I am not so concerned with the speedometer being
    off, I do wish the MPG gauge was more accurate.
    The MPG gauge should be an easy soft ware
    fix for a competent computer pro.
  7. tonyrenier

    tonyrenier I grew up, but it's still red!

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    My speedometer is off 2 mph according to two GPS devices. My mpg is off 2.1 for 35,000 miles consistantly per my calculator. (of course, I haven't check my odometer, I think that has to be right on by law-very interesting.
  8. F8L

    F8L Protecting Habitat & AG Lands

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    The car's computer knows your real speed. Using a Scanguage, which plugs into the OBDII port, shows a consistent 2mph less than the speedo. My GenII did the same thing.
  9. jdcollins5

    jdcollins5 Senior Member

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    I did a test with SGII and determined that at 60 mph and a MG1 speed in the neighborhood of 7000 RPM, and knowing the gear ratio between the motor and the wheels, that the true car speed is 58.6 mph. The SGII bounces between 58 and 59 mph.

    I have not tried this test at other mph.
  10. ProximalSuns

    ProximalSuns Senior Member

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    Haven't been able to get it up to 100 at the various radar speed signs the police put up recently but every one of them consistently shows the Prius reporting a higher speed than the radar shows.

    Usually about 2 mph difference with the Prius saying it is going faster than the radar claims.

    That's one way to get better mileage on the trip computer.

    It also might explain why Prius driver's say they haven't changed their driving habits but are suddenly getting tailgated all the time. They are actually driving 2 mph slower than they used to and with a digital speedometer, the are more accurately, though inadvertently going slower.
  11. F8L

    F8L Protecting Habitat & AG Lands

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    This is not Prius specific. Most cars have this error. My Corvette did and so did my girlfriend's Rav4 until she purchased different sized tires. :)
  12. ProximalSuns

    ProximalSuns Senior Member

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    Without digital speedometer, hard to know. It would explain the consistently lower mpg reported by Prius numerologist vs. Prius computer and the tail gating issue most new Prius drivers encounter.
  13. ETC(SS)

    ETC(SS) Aspiring Hypocommuter.

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    IIRC, industry norm is +/- 3-percent.
    The "Published" reason is to account for tire wear, but most makes don't mind shaving 2-3 percent off of their warranty liability and having a good reason to do so.
    Japanese makes are somewhat notorious for favoring the house with speedo errors, and the MFD on Priuses are no exception.
    Interesting enough, when I got my company Prius, I did some measurements with 2 GPS units and ran a measured 100-mile course and the speedo was almost dead bang accurate, Like within 0.5-percent.
    The MFD on the other hand had a 3-percent bias favoring the house.
    Since I always hand-calculate tank to tank, I pretty much ignore what the MFD displays.

    My Truck is within a half percent on speed (no MFD) and my Envoy has always had a minus 3-mph error. The CFO's car is too new for me to have any stick time in, but I presume that it's within the industry norm.

    As they say....YMMV! ;)
  14. fuzzy1

    fuzzy1 Senior Member

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    The UN document posted here on occasion is a trade regulation for the EU and parts of Asia. The list of covered countries includes nothing in North / Central / South America. PC readers in covered countries often report larger offsets than do owners of US-market cars.

    In the U.S., the speedo bias appears to an industry standard, of which Toyota is a part, and from some state laws forbidding speedometers to read low. Because a population of measurement devices will always have some error spread, the average must be biased high to keep the low end of the distribution in compliance.

    Of the four cars I measured, my Prius (2010, haven't yet checked the 2012) was the second most accurate, at about 1-1.5mph error. Our Suby is off by about 0.5 mph. A Honda was off by 2, a Ford by 4.


    Exactly where does speed show up in the MPG formula?

    It doesn't, and is thus irrelevant to the Prius MPG error.

    This has already been discussed ad nauseum in PC. Speedometer and odometer errors are separate. My Prius odometer read 0.2% low, not high.


    Nope, because the Prius isn't meaningfully different than the other cars out there.


    Any manufacturer still doing this is rolling out the red carpet for the product liability lawyers. Years ago I received the class action notices for these suits against Honda and Subaru. Both settled for a 2% odometer extension on the warranty. I had previously measured against survey markers, and neither of my cars subject to these suits had that much error.
  15. jdcollins5

    jdcollins5 Senior Member

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    I have checked my odometer against the mile markers on both I-40 and I-85 in NC and they are within 0.1 mile of the mile markers, most of the time right on the money.
  16. alfon

    alfon Active Member

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    Odometers have to be accurate and normally read
    under. On our Prius 100 miles true traveled will
    read about 99.5 miles or so.

    The reason being so it will not effect warranty
    coverage.
  17. Mr Incredible

    Mr Incredible Chance favors the prepared mind.

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    My truck and Z28 speedometers are spot-on. I don't think I've checked Wife's Prius since I never really drive it and don't feel like hooking up the S.G. and losing all my input.

    It isn't the end of the world, I suppose, to have to remember that I'm 1mph slower than the C's speedo. Close enough for me.
  18. Ridder

    Ridder Member

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    My Gen3 has a 10% offset ....that has been put in by Toyota deliberately.
    The SC2 reports the right speed, let's say 60, and the display shows 66.....

    I really don't like that.....when I'm doing 100 km/h (speed limit) on the speedometer, then trucks are tailgating me....and Prius drivers get a bad name.
    So when I wanna drive 100 km/h I have to go 110 km/h on the display.....
    I called Toyota EU about it and they said it was error margin.....yeah sure.....10%!
    But it gives you nicer MPG numbers.....
    And of course there's nothing they can do about it....
  19. Mr Incredible

    Mr Incredible Chance favors the prepared mind.

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    It's easy enough to get right when you buy the next set of tires. I got snow tires in a different size and the mph differential was easily cut in half. Next time I get tires I'll get that size. Couldn't say what it is at this time, they're outside and I'm not.
  20. ztanos

    ztanos All-around Geek!

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    You would think so... but when you factor in that the speedometer is already almost 2mph slower than your actual rate of travel it gets exasperated.
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