Interesting mpg displayed on Electronic Traffic Speed Monitor

Discussion in 'Gen 3 Prius Main Forum' started by lovemypriustoo, Jun 30, 2011.

  1. lovemypriustoo

    lovemypriustoo Junior Member

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    Interesting -- This morning I was driving 36 mph (in a 35 mph zone). As I approached the electronic traffic speed monitor on the side of the road, it displayed my speed at 34 mph (instead of 36 mph as indicated on my speedometer). Has anyone else noticed this type of discrepancy? I would rather be going slower, in any event, especially when you are being followed by a cruiser ...
     
  2. spiderman

    spiderman wretched

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    Yes, known issue... You fuel economy display is also 4-6% more generous then actual.
     
  3. V8Cobrakid

    V8Cobrakid Green Handyman

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    a lot of cars do this... a lot... for example, my g/fs mini says you're going almost 5mph faster than you are...
     
  4. F8L

    F8L Protecting Habitat & AG Lands

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    Like Spiderman stated, it is a known issue. I did notice however, that while my car normally shows a 2mph difference between the speedometer and actual, when I installed brand new tires (215/45/17) the speedometer is now corrected. I checked it 3 times yesterday via Scanguage and GPS and it was exactly the correct speed. The extra new tread increased the rolling diameter of the tire and fixed the error. Eventually as the tread wears down the error will come back.
     
  5. lovemypriustoo

    lovemypriustoo Junior Member

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    Thanks F8L, that's interesting to note about the tire tread. BTW, my mpg displayed and hand calculated only differs by 1 mpg so far (which I think is pretty good).
     
  6. F8L

    F8L Protecting Habitat & AG Lands

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    I believe the GenIII generally only suffers from a 1mph error where the GenII was a 2mph error. At least the GenII here in the states. I believe this is due to our odd tire size (185/65/15). The 195/60/15 should be even worse but I've not seen any reports from other countries. Both the 195/65/15 and the 215/45/17 are larger in diameter than the 185/65/15 so the speedometer slows down a bit.
     
  7. tonyrenier

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

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    Mine's been generous by about 2 mph since new. Varified with several GPS, Scangauge II and the Roadside monitors as well. I've noticed this in other cars too.
     
  8. stream

    stream Senior Member

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    As discussed in a few previous threads this is common in most/all cars, due to laws in certain countries (Germany as an example) that dictate that a car's speedometer must be within X% accurate, but never display lower than actual MPH. So manufacturers err on the high side.
     
  9. twittel

    twittel Senior Member

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    The other issue to consider is whether or not the radar units are callibrated correctly by the manufacturer or judicatory. My Prius shows 1MPG higher than actual radar reading in my neighborhood.
     
  10. qbee42

    qbee42 My other car is a boat

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    This is the key issue. It comes from U.N. regulations involving the sale of cars between countries. Speedometers are allowed to miss by quite a margin on the low side, but not understate. From Wikipedia:

    Tom
     
  11. David Beale

    David Beale Senior Member

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    Regardless of laws, do you really expect a 1% accuracy on your instruments on a >$40,000 car?
    Dreamer!
     
  12. Corwyn

    Corwyn Energy Curmudgeon

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    Regardless of price, do you really expect a 1% accuracy on a machine for which the relevant surfaces are ablative? Tires wearing out is going to produce a larger variance than that.
     
  13. rxlawdude

    rxlawdude Active Member

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    But since you're using the odometer to measure the numerator, you're really only measuring the optimism of the fuel efficiency display. I guess the question really is if the indicated speed is off on the high side, doesn't that mean the odometer would be off by that same percentage?

    The lawyer in me wonders the impact on warranty claims.

    Bob.
     
  14. David Beale

    David Beale Senior Member

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    I haven't seen much correlation between the odometer and the speedometer. I will admit I haven't -fully checked- the Prius. I use my GPS as my standard. It's specified at 0.1% accuracy. Pearl's speedometer is usually within 1 km/hr of the GPS. It read low when the tyres were new, now it reads high.
    And yes, Pearl will get new tyres this year, probably in a few months.
     
  15. Rocco42

    Rocco42 Member

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    Speed displayed on the speedo has forever been off. It has to do with errors in the machinery, the tires, and a lot of other things.
    Even the thing that displays your speed that the cops setup could be wrong. If you approach it on an angle it'll display differently than if you approach it straight on.They never calibrate the thing, so they cannot use it in court.
    When they use speed guns, they have to calibrate them with, get this, tuning forks, at a predetermined distance before they can go out in the field with them. At least that's how it was about 10 years ago, when I did speed studies as a traffic engineer. We would often set speed limits by what 80% of the vehicles were doing on a decent sized straightaway. I know it sounds stupid, but that's what the Manual for Uniform Traffic Control Devices states, so we use it.
     
  16. xs650

    xs650 Senior Member

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    No, the odometers are as accurate as they can reasonably make them considering they are based on rotations of a tire. Speedometers intentionally read high so owners can;t legitimately blame speeding tickets on the manufacturers.

    If you hook up a device like a ScangaugeII and compare it to a GPS you will see that a car typically knows how fast it is going but exaggerates that number for the speedometer reading.
     
  17. David Beale

    David Beale Senior Member

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    Not quite. -Modern- "speed guns" aren't affected by angle or motion of the gun. They compare the object of interest to the ground or surrounding objects.
    You ARE correct that there is a specified accuracy. It's usually in the order of +or- 5% -after- calibration. Many charged motorists have had the charge thrown out of court due to failure to calibrate, speed within accuracy, etc.

    Here in Alberta they allow in law 10 km/hr to compensate for the inaccuracies of the apparatus. So you have to be going -more- than 10 km/hr over the posted limit to be fined. You can still get a warning though. In Edmonton, we have lots of "speed on green" cameras combined with red light cameras. They allow 14 km/hr before flagging a suspect. Even at that, they were deemed too inaccurate by a court of law and the govt. had to return more than $3 million in speed fines (I was one of the people who got the money back). This after one guy was charged with going as I recall 170 km/hr -in traffic- with all the other vehicles going about the same speed (two photos are taken and the movement of the suspect vehicle vs the others is used to flag it). My ticket was for 74 km/hr in a 50 zone and I know for a fact I was going 60 (passing a bunch of cars going 40). A $60 fine isn't worth my time and effort to fight. But they got their comeuppance anyway. I'm framing the letter that came with the cheque.
     
  18. tumbleweed

    tumbleweed Senior Member

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    If you use a ScanGauge check the MPH display. The speed from the ODB is pretty much right on. The speed displayed by the speedometer is 3 or 4 % fast (using OEM tires) to insure compliance with the UN regulation previously mentioned. So the car "knows" how fast it is going and the speedometer error is by design.
     
  19. rxlawdude

    rxlawdude Active Member

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    Respectfully disagree. In the past five years alone, there have been several class-action suits settled with manufacturers tacking another 3-5% onto warranty durations as a result of overstating distance traveled.

    Now, if you can show that over a statistically significant sample size that the odometers are in fact accurate (while speedo is not), and that the scanguage (accurate) speed is what drives the odo, then you'd have a valid point.

    It seems anomalous that if the speedometer reads 60mph and you drive exactly 60 minutes, the odometer will show something less than 60 miles driven.
     
  20. Troyroy

    Troyroy Member

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    My Prius has always been 2 MPH off since new. Can't see how tires affected it new.....Yes, warn tires will spin faster, causing the MPH to show the wrong reading. But as a new car, mine was always off by 2 MPH, check with my GPS.
     
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