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calculated vs. computed MPG anomaly?

Discussion in 'Gen III 2010+ Prius Fuel Economy' started by draheim, May 12, 2010.

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  1. draheim

    draheim Member

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    I realize the computed MPG on the display is typically off from (and usually higher than) actual calculated MPG using gallons to fill up and miles driven on previous tank...

    But I found a surprising anomaly (?) on my second fill-up: the displayed MPG is higher on my second tank than what I got on my last tank—but the calculated MPG is lower. This is from filling my tank at the same gas station, not topping off, and taking careful notes.

    I bought the car on 3/27 (with maybe 6 miles on the odometer?) and since the dealer filled up the tank, my log is blank for that fill-up.

    On 4/14 I put 7.984 gallons in and recorded that I had driven 338.5 miles on that amount (for a calculated actual MPG of 42.397); display MPG was 43.4. I then reset the Trip A meter.

    On 5/11 I put 8.139 gallons in and recorded that I had driven 335.1 miles on that amount (for a calculated actual MPG of 41.172); display MPG was 44.0. I then reset the Trip A meter.

    Based on other threads here I would have expected the display MPG to be consistently lower than actual MPG; but I did not expect the actual MPG to be lower on the second tank even though the displayed MPG was higher!

    Can anyone help explain why this happened? Am I calculating this wrong? Or maybe the 6 or so miles on the odometer when I bought the car throws these numbers off? My understanding is that I should take my current mileage and divide it by how many gallons it takes to fill the tank on that day—because that is how much gas you actually used to get to that current mileage. Is that not correct? (Disclaimer: I am not a math whiz) Thanks.
  2. rrolff

    rrolff Prius Surgeon

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    Re-read the thread. The car always shows better mileage (than calculated) = 5%+
  3. jdcollins5

    jdcollins5 Senior Member

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    I think the problem is going to be with the first dealer fillup. You do not know how much was in the tank to start with. After you get a few more fillups you should get more consistent readings.

    Another problem may be between different gas stations and pumps with different shut-off points.

    After about 8 months, my calculated mpg is very consistent with a difference between the MFD displayed and my calculate-at-the-pump of +3.3 mpg. This is fairly close to what others on here are seeing.

    My average is about 51 mpg displayed and 48 mpg calculated.
  4. draheim

    draheim Member

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    I know that it's always higher; what I'm saying is that the discrepancy widens in both directions on the second tank.
    Tank Display Actual Discrepancy
    1 43.4 42.397 -2.36%
    2 44.0 41.172 -6.87%

    I was expecting the display to consistently be higher by a certain percent. The variation between real and computed MPG is more than 4% from the first tank to the second tank.
  5. rrolff

    rrolff Prius Surgeon

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    The dealer tank is really a random data point if you're looking for accuracy... Who knows how it was filled...
  6. Felt

    Felt Active Member

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    I fully understand there could be many other causesfor what I am experiencing other than design intent .... But:

    I have 12,500 miles on my G3, and one winter. My records reveal late summer last year, my indicated and actual (mpg) were about 2+ mpg off. During the winter, the spread was about 4+ mpg. I am suspicious that Toyota programmed the computer to indicate a slightly better efficiency during the winter when the actual mileage was falling. Now that it is warming, the indicated has increased, as has the actual, and the difference is getting closer to what I experienced in the late summer last year.

    Said another way ... if I plotted the indicated ... the line would dip in the winter, but remain "somewhat" flat. If I plot the actual, the line would take a distinct dip in the winter. Whatever the cause ... I am grateful that the actual numbers are climbing.
  7. draheim

    draheim Member

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    Interesting... since the car has an outside thermometer Toyota could have programmed the computer to adjust MPG based on detected outside temperature... Or it could be just that the car actually burns more fuel in winter to warm up and adjusts computed MPG accordingly to compensate...
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  8. jdcollins5

    jdcollins5 Senior Member

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    I have tracked my displayed versus calculated mpg for eight months now, from late summer through winter and now spring. My difference between the two numbers has been a consistent 3.3 mpg. I track this in an Excel spreadsheet with a graph and the two mpg lines track each other with a 3.3 mpg gap between them.
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  9. fuzzy1

    fuzzy1 Senior Member

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    If the lot boy topped off the tank, or used a pump that shuts off a quart later than the pump you use, or was parked on a pad sloped away from the pump, your first fill would look artificially good.

    Because of this uncontrollable and unknown variation, your own first visit to the pump doesn't really count.
  10. lunabelgium

    lunabelgium Member

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    What's the true ?
    What the display MPG show or what you really fill up the tank ?
    With time going on you'll be able to determine a valid % of error.
    This is the subject of sharp discussed on this side of the Atlantic - LOL -.
  11. alfon

    alfon Active Member

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    27,000 plus miles, consistently off 3 mpg average. sometimes less, sometimes more.

    alfon
  12. quantumslip

    quantumslip Member

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    i know what you mean; i have seen something similar with two relatively recent fillups:

    05/06/2010: 52.2mpg comp, 49.80mpg calc, ~4.8%
    04/30/2010: 54.0mpg comp, 49.87mpg calc , ~8.3%

    Usually variations are close to 5%, but this one really puzzled me. I guess there will be outliers, but I was really expecting 50+mpg with the 54.0mpg one. It did seem a little high in the first place (got it while going 70mph from College Station to Houston and some driving to/from work at around 65mph). I had a lot of good tailwinds then so that might have affected the computer calculations.
  13. snoctor

    snoctor Member

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    I've had my model III 2010 Prius for 9 months and driven over 12,000 miles. I've kept track of gasoline purchases (27 tanks of gas filled so far), gallons purchased, actual mileage and the car's computer estimate for each tank. I'm averaging 52.2mpg over 12,600 miles, actual mileage. The car's computer estimate is 5.3% high, at 55 mpg over the 12,600 miles. As others have noted the car's estimate varies from tank to tank. In my experience the car's computer estimate varies quite a bit, from 2% overestimate, to 12% overestimate. In the case of the very high estimate the car calculated that I'd gotten 58.2 mpg but I calculated 51.8 actual mpg. The car's estimated mileage has never been lower than actual mileage of course, the closest the car's estimate was to my calculated mileage was 2.1% over. While there is quite a bit of variability (2% to 12%) part of that variability could depend on how I fill up the tank - did I fill up the tank to the same exact point each time - probably not. I always fill till the gas pump shuts off. Best I can do I suppose. But over the long haul things like that should average out when calculating actual mileage and car's estimated mileage.
  14. psu77

    psu77 New Member

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    My car is one year old and has about 13,000 miles. The calculated mileage has always been about 3 mpg higher than actual. Then the last couple of months I noticed that the calculated mileage was almost exactly equal to actual. What caused the difference?

    Here's my theory. Previously I always ran in Eco Mode and tried to pulse and glide for better gas mileage. A couple of months ago I switched to using Power or Regular mode, drove the car with less pulse and glide and now the mileage calculation is more accurate. My actual mileage has stayed almost exactly the same - about 53 mpg.
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  15. lunabelgium

    lunabelgium Member

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    Very interesting. I'll keep a glance at this point.
  16. mmichaell

    mmichaell New Member

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    For me, I don't P&G, I just drive it. Real mpg is 54mpg now, computer normally reads 57.5mpg, I still get a 3.5 mpg difference.
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