Speedometer Error about 1 MPH?--Some evidence

Discussion in 'Gen 3 Prius Technical Discussion' started by kgall, Mar 5, 2010.

  1. kgall

    kgall Active Member

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    Speedometer/"Trip Average" speed differ by about 1 MPH?

    I've been trying to get figures on mileage at various speeds by setting cruise control to the desired speed and then resetting "Trip A". I will then adjust the displayed MPG down by what I've figured as my average displayed MPG error (about 2.6 mpg). For a lot of reasons, I don't have any useful MPG vs. Speed data yet, but I do have something on the question a number of us have asked on how accurate the Prius speedometer is:

    When I set the cruise control and achieve the desired speed, then start a new Trip A, the Average Speed displayed for trip A has always been 1 MPH slower than the Speedometer reading, for speeds from 60 to 72 MPH.
    For 76 MPH today, the Average Speed displayed was 74 MPH, or 2 MPH less--I wonder if this might have been a rounding error, as I was trying to hit 75 MPH so it might have been a "slow" 76.

    Which is right? I don't know.
     
  2. Joe166

    Joe166 New Member

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    I hooked up my Garmin GPS one time and the speedo was right on. I don't think 1 mph is an error. That is well within the variations caused by heat, tire expansion, etc.
     
  3. JimN

    JimN Let the games begin!

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    MY RCC is 1mph higher than the speedo. The Garmin speed is also off 1mph compared to the speedo but I don't remember if it is high or low.

    If you want to see error buy a car in a country that follows the UN guidelines. The speedo is off 10%.
     
  4. tumbleweed

    tumbleweed Senior Member

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    Speedometers usually read a little fast, mine is off 2 MPH at 70. There is a U.N. regulation that all car companies abide by that says the speedometer can't read slow but can be up to about 5% or 10% fast if I remember correctly. Most, like my Prius, are off by 2% or 3%. There is a thread discussing that on this site somewhere that gives a link to the U.N. regulation, I think it dates back to the 1950s. Others, such as Joe166's, are pretty close but with the original tires probably none of them are slow.

    The car seems to know how fast it is really going, if you read MPH from the data bus with a Scan Gauge it reads the correct speed, at least mine does. I think the odometer readout is pretty close also.
     
  5. Joe166

    Joe166 New Member

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    Can somebody explain to me what the hell the United Nations is doing getting involved in cars? I guess they have solved all of the rest of the important problems in the world and they can now get down to the little ones? I guess I missed that movie.
     
  6. tumbleweed

    tumbleweed Senior Member

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    I understand your point exactly! I can't give an answer outside the political forum for fear of offending some of the U.N. advocates. :D
     
  7. David Beale

    David Beale Senior Member

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    It's a UN guideline because nobody else in the world wanted to or had any authority to ask that speedometers actually read correctly within some standard. If you think the USA can regulate what other countries produce, you've been watching too many comics. Countries can only regulate what is sold within their borders. As most want tourism, they allow vehicles from other countries to enter and operate within their borders. This was done first in Europe, where there are -many- countries to deal with. In our myopic universe (North America) it wasn't an issue until the "big three" pee'd away their monopoly on car sales and foreign made cars became common.

    If you do use your GPS to check the speedometer for accuracy, be advised, if you are ascending or descending a hill it can throw the GPS off for a minute or so. Otherwise, it's more accurate than just about any other measurement method, short of laser rangefinders.
     
  8. mainlin

    mainlin Junior Member

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    Checked my '10 V speedo with TomTom, police radar, and my Honda. The Honda always shows + 1 m.p.h., the Prius + 2-3 m.p.h., so I always subtract 2 m.p.h. to find my actual (or drive + 2 to achieve desired speed.)
     
  9. sipnfuel

    sipnfuel New Member

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    My GPS says that the speedo is high by 2-3 mph when the speedo reads 70-75 mph
     
  10. fuzzy1

    fuzzy1 Senior Member

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    I don't believe those UN standards carry any weight in the U.S. market. However, some U.S. states have similar regulations.
     
  11. cwerdna

    cwerdna Senior Member

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    European?

    Perhaps you guys are talking about European regulations and not the UN?

    I posted a C&D article on speedo and odo mileage at http://priuschat.com/forums/gen-ii-...oting/28808-odometer-accuracy.html#post368671 before. There's also an easy to miss .pdf link from that article at http://www.caranddriver.com/var/ezf...lication/1e1b33f85f31df61e4830f32f82a5a71.pdf. Keep in mind this article was written in 2002.
     
  12. F8L

    F8L Protecting Habitat & AG Lands

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    The GENII shows a 2mph error when compared to ScangaugeII readings as well so I am inclined the believe the GPS reading is correct.
     
  13. johalareewi

    johalareewi Member

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    Over here, the Prius speedometer reads 75 when the GPS shows 70. But the odometer is spot on so the Prius knows exactly how far it has gone (and in what time) so it should be able to get the speed spot on too. The law here that says the speedo can be up to 10% over but mustn't be under so if you are doing 70, the speedo must be somewhere betwen 70 and 77. Most cars aim for the top end of the range to be extra safe.
     
  14. twittel

    twittel Senior Member

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    My speedo is +1 when measured against police radar in a neighborhood setting where posted speed limit is 35. Regarding the OP statement about rounding up errors, what's being rounded; i.e. the 1/10 miles? I assume there is never any rounding up or down with a vehicle odometer and/or trip meters.
     
  15. qbee42

    qbee42 My other car is a boat

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    The Prius speedometer is a digital unit. Therefor there must be either truncation or rounding errors. It is impossible to avoid, unless you have an infinite number of digits.

    Tom
     
  16. qbee42

    qbee42 My other car is a boat

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    Re: European?

    It's sort of both: United Nations Economic Commission for Europe (UNECE) Regulation 39. It was intended to ease international trade in automobiles.

    Tom
     
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  17. rrolff

    rrolff Prius Surgeon

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    This is not true.

    UNECE Reg 39

    The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1 ) and the true speed (V2).
    0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h
     
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  18. ken1784

    ken1784 SuperMID designer

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  19. demiller9

    demiller9 Junior Member

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    A ScanGauge can display the speed, taken from the ECU. The speedometer doesn't agree with the ScanGauge, always reads higher than the SG. This shows me that the speedometer has been 'tweaked' to follow the UN regulations. (The ECU speed is very close to the speed my TomTom GPS shows).
     
  20. Joe166

    Joe166 New Member

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    Wow! IF a country were to actually sign that garbage, then all they have to do if they feel like non conforming is to let the other signatories know that they aren't going to go along with it. What an organization. Maybe they will send the French Army with blue berets in to force compliance.
     
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