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Which is more Accurate?

Discussion in 'Gen 2 Prius Fuel Economy' started by Kcissem, Jul 30, 2008.

  1. Kcissem

    Kcissem New Member

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    Which is more accurate the MFD MPG or the calculated MPG? Reason i asked is because my mfd said 48.2 MPG average but based on the first fill up, the calculated MPG is 41.1. So which to trust more?

    I would figure the MFD is more accurate as Calculating based on ammount of gallons filled can be variable at times due to the bladder. For instance first fill up took 12 gallons before the automatic shutoff valve hit. The only explanation to putting more than the rated tank size is due to bladder expansion. I was only expecting to put in about 10 to 10.5 and with that the calculated MPG would have been almost spot on with the MFD MPG. Of course this is just one fill up, calculated MPG will probably average out in the end
     
    #1 Kcissem, Jul 30, 2008
  2. bushface

    bushface Junior Member

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    I use lifetime mileage divided by total gas purchased. This has to be dead accurate after ten or more fillups. Checking one tank at a time with a blader is not trust worthy. I dislike the bladder part of my car and the limited adjustable seats but like everything else.

    10312 miles divided by 209.562 gallons equal 49.2 mpg.

    That is my 2 cents worth.
     
    #2 bushface, Jul 30, 2008
  3. Kcissem

    Kcissem New Member

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    Yes, i figure it will average out in the end, i am currently creating a forumla spreadsheet to track on an individual, Lifetime, Seasonal, and Brand basis tracking anything from season, miles, avg mile per tank, amount of tanks, gallons per tank, MFD Mpg, Calcuated MPG, cost, cost per gallon avg, cost per MFD mpg mile, cost per calculated MPG, Mile etc. etc. Am about 80 percent done with the spreadsheet, I may put it up for download eventually if anyone is interested and or don't know how to use forumlas within excel.
     
    #3 Kcissem, Jul 30, 2008
  4. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    MFD. Hands down. Every time.

    There are many many MANY threads debating the accuracy of hand calculating. In the end, from tank-to-tank you just can't do it. Lifetime average, of course. Tank-to-tank; never.

    THe Prius knows how many times the tires rotate and how many miles you've driven. Likewise, it knows when the engine is running and how much gasoline is being squirted into the piston. Therefore, it knows better than you exactly how many miles you've driver per how many gallons you've consumed.

    And don't even get started on the bladder.

    And if you've interested in a spreadsheet, hit the link in my signature.
     
    #4 TonyPSchaefer, Jul 30, 2008
  5. effwitt

    effwitt Paparazzi Magnet

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    The MFD display might be off by a few percentage points, but if it's consistently off by the same percentage, does it really matter? The size of a gallon and the length of a mile are arbitrary anyway.

    If my last tank was 45 mpg and the current tank is 50 mpg because I started taking a different route to work, doesn't this tell you what you really want to know?

    Comparing your mileage against someone else’s can be like comparing apples and oranges. Different climates, tires, tire pressures and differences in the average speed of traffic can make a huge difference in your numbers. Gas pumps aren't exactly precision instruments either.
     
    #5 effwitt, Jul 30, 2008
  6. Kcissem

    Kcissem New Member

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    I wasn't comparing mileage to someone else i was comparing my own mileage based on the MFD mpg and manual calculated MPG.
     
    #6 Kcissem, Jul 30, 2008
  7. WayneF

    WayneF Junior Member

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    My own recent experience with my 2008 is 3632 miles on 78.05 gallons, or 46.54 MPG. The display said 47.7 MPG overall for same miles. That is close enough for me.

    These miles were very little city driving, mostly 70MPH highway and interstate driving with air conditioner on, in 90 to 100 degree weather. And a good part at 75MPH, but also included a few hundred miles over more than a dozen 10,000 to 12,000 foot passes on a Colorado vacation. The MFD hardly ever varied, regardless of the conditions.

    Regarding mountain climbs, I found it was greatly more pleasant to turn off AC, and slow to 30 or 35 MPH to make it up the long climbs. Because otherwise, the battery is only good for a very few minutes, then it is gone, and that is trouble. But slowing to 30 MPH and turning off AC lets the motor/generator keep up at slow speed, even on the climb, so that the battery never quite goes to absolute zero. Which then avoids the very high RPM mode on the little ICE... It acts more normally, but just slow.

    However, the Pruis easily made it up Pikes Peak, very uneventfully - given that it was mostly at 15MPH (same as all the big boys there). It is still economical, but the Prius is quite underpowered for such mountain driving, as the battery duration is totally useless then.

    Nevertheless, I love my little car, it is absolutely fantastic on regular highways and in the city.
     
    #7 WayneF, Jul 30, 2008
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