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TRIP / ODO accuracy

Discussion in 'Gen 3 Prius Main Forum' started by Sergio-PL, Mar 27, 2012.

  1. Sergio-PL

    Sergio-PL Member

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    Hi!

    I did short search via forum. I found a lot of threads about speedometer accuracy and MPG readings. But does anybody checked the ODO accuracy?

    Today I did full refill with TRIP A showing 700.3 km. I always write down ODO on my fuel tickets and it matches with TRIP A. Result is 700km.

    Then TRIP A shows reading of 4.9 l/100km vs 5.3 l/100km calculated. And generally it's within the range of 7 - 8% reported by others.

    But I tried to play a bit with numbers and:
    5.3 l/100km is 8% more than 4.9 l/100km
    Max speed 180 kmh is when speedo shows 194 kmh => ~ 7.7% less

    And until now everything is OK, but then:
    For 700 km (TRIP A) I filled up 36.92 litres. If 4.9 l/100km I should fill up only 34,3 litres.
    If I would fill up with 34,3 litres it would last for ~647km with 5.3 l/100km.

    I put all those numbers into Excel and one interesting result shown up.
    700 km * (107,7%) = 754,4 km
    36,92 l / 754,4 km * 100 = 4,89 l/100km

    It would mean (if ODO is accurate) that MPG calculation is based on the innacurate speedometer reading not real traveled distance. :confused:

    Single tank is not enough to proove anything but I'm sure that most of you have more readings to check it. I've tried it on few results from Spritmonitor and within 1% my last calculation (speedo error * distance traveled) seems to give correct results (matching BC).

    Can you check by your car (within applicable speed limits) the following numbers:

    1. Take steady speed, flat terrain. More round speed the better. While keeping in for some time check speed returned by GPS device. Notice both.
    2. Calculate speedo error using formula X = 1 - (GPSspeed / PRIUSspeed) * 100.
    3. Take readings from full fill-up where you have reported (BC), calculated consumption (Calcd) and distance (TRIP).
    4. Calculate BC distance using formula BCdist = TRIP * (1 - X)
    5. Divide fuel consumed by BCdist and compare it against BC. It should be the same number within 1%.

    All my calculations are based on EU 2011 Prius with stock 17" wheels at manufacturers recommended pressure.
     
    #1 Sergio-PL, Mar 27, 2012
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  2. F8L

    F8L Protecting Habitat & AG Lands

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    Something to keep in mind when comparing your calculations with those of other members. The Prius Five comes equipped with a 215/45/17 tire which specs to approx. 844 revs/mile. The other Prius models comes equipped with a 195/65/15 tire which specs to approx. 829 revs/mile. As far as I know there are no computer changes in either model to acknowledge the change in tire size. Thus the Prius Five speedo should read higher than the lower models. My model Three reads approx. 1mph-1.5mph too fast compared to GPS and 2mph too fast compared to Scangauge.
     
    #2 F8L, Mar 27, 2012
  3. jdenenberg

    jdenenberg EE Professor

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    Yes, the revs/mile (goes up as they wear) of your tires directly affects ODO calibration. You can use your GPS to check ODO (and speedometer) calibration.

    JeffD
     
    #3 jdenenberg, Mar 27, 2012
  4. JimN

    JimN Let the games begin!

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    The European speedo conforms to UN specs which are more liberal than the US spec. My speedo is most likely to read 1 mph faster than my Garmin GPS. The reported fuel economy is ~6% higher on average per tank than the calculated economy. I have not noticed any discrepancy in the odo.

    How do you account for the variance in fueling? What is full? When a pump shuts off early or late a little bit of gas will have a pronounced effect.

    An example:
    600 miles, 10.000 gallons pumped for 60mpg, $35.90
    If the attendant rounds it to $36.00 that's 10.028 gallons for 59.8mpg

    With 10 gallons an 8% difference is 0.8 gallon, $2.87 worth today. I believe there is enough headroom in the tank to accept this amount without spilling.

    Alternating high & low FE readings would indicate the pump is the source of the problem.
     
    #4 JimN, Mar 27, 2012
  5. ImeanGreen

    ImeanGreen Prius v Five BP Brigade #236

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    I don't know if we could use the GPS as a standard. My honda and my toyota van shows the same thing with the speedo faster than a mile or two than what my garmin GPS says. I think all these numbers are estimated and not really actual. Meaning it gets close to the numbers as much as possible. The distance to fueling also depends on the speed you are traveling. Just too many variables IMO. As long as it is close give or take 1-2 miles, I'm very satisfied.
     
    #5 ImeanGreen, Mar 27, 2012
  6. Sergio-PL

    Sergio-PL Member

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    Hi. Full is when I fill up until first click. No rounding, no topping up. I always use self service stations, most of the time it is the same station near home and same pump if possible (ie. yesterday someone took a coffie and locked pump for 15 minutes...). This method worked fine for 3 years and 3 different cars.

    So I don't have roundings. My numbers are also higher (in numbers) because of litres and kilometers so less rounding occurs.

    Fuel here costs $5,35 per gallon... I'm not sure if I should put a smile here... If I don't mess with currency our German friends pay about $7 for gallon.

    I asked about ODO accuracy because I'm charged for km's by company so big numbers here would mean direct loss in money. ;)
     
    #6 Sergio-PL, Mar 28, 2012
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