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Paying for air/vapors at the pump?

Discussion in 'Gen 2 Prius Fuel Economy' started by mikewithaprius, Mar 4, 2011.

  1. mikewithaprius

    mikewithaprius New Member

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    This evening, with Wentworth (the Prius) in the shop for preventative fixin' up of the water pump/motor mount from various TSBs, I was driving another car that averages 25 mpg ordinarily. I had just filled it up the yesterday, so I was curious what would happen if I tried to drive it for good mileage.

    I drove 7.5 miles this evening, and filled up at the Shell station right next to my destination. It put in .7256 gallons. If I say I also drove home .4 miles from the gas station yesterday, that's paying for .7256 gallons over 7.9 miles, or 10.89 mpg.

    This is obviously absurd, and even if I only got 25 mpg, it overcharged me by .4 gallons or so. I feel I likely got into the 30s no problem, which would mean an overcharging of about a half gallon of gas.

    I know you've all heard of pumping slowly at the gas pump, and some publications online I've read (online...i.e. taken with a grain of salt) say this is useless, or not economically significant to the consumer. My question is whether this sort of huge overpayment apparently stemming from the first parts of a fill-up (although surely unnoticed when filling the whole tank) are common, if they would be uniform from station to station, or if some stations would have better equipment to prevent this, or anything else you wish to comment about. Or is there the possibility of inaccuracy simply because of the short distance driven?

    Besides paying an extra $1.75 or so every fill-up, it hugely affects calculated mpg for a high mileage vehicle like the Prius. For a regular car, it would "reduce" your calculated mileage over 200 miles from 20 mpg to 19 mpg. For a Prius truly getting 60 mpg over 400 miles, it reduces it to 55 mpg.

    Just fyi, I did pull the trigger pretty slowly, too. Any comments/insight are appreciated.
     
    #1 mikewithaprius, Mar 4, 2011
  2. cthindi

    cthindi Member

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    I think it is the bladder in gas tank the cause of this issue. I would not trust consumption after driving so little in any car for that matter, one not having bladder either.
     
    #2 cthindi, Mar 4, 2011
  3. mikewithaprius

    mikewithaprius New Member

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    I thought it might be the short bit of driving - but half a gallon off seems like a lot.

    This was a Dodge Neon, by the way, no bladder as far as I know.
     
    #3 mikewithaprius, Mar 4, 2011
  4. uart

    uart Senior Member

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    Hi Mike. The problem is not the gas pump overcharging you (well not that we can prove with this data). The problem is simply that the exact point where the gas pump cuts out and exactly how the fuel is sitting in the tank is somewhat variable. You cant rule out the possibility the you pumped in an extra 1/3 gallon during the second fill. Even this tiny discrepancy in the fill volume would change your MPG calculation from 10MPG to 20MPG.

    The bottom line is that small refill volumes are just incredibly unreliable for calculating MPG's.
     
    #4 uart, Mar 4, 2011
    1 person likes this.
  5. mikewithaprius

    mikewithaprius New Member

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    Thank you both, consensus is short distance is just unreliable for calculating.
     
    #5 mikewithaprius, Mar 5, 2011
  6. bisco

    bisco cookie crumbler

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    in massachusetts the pumps are all tested regularly for accuracy by the local board of weights and measures. it all started at the butcher shop years ago when the butcher chopped his thumb off by accident while carving up a heffer and left it on the scale by accident. (so he said)
     
    #6 bisco, Mar 5, 2011
  7. JimN

    JimN Let the games begin!

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    11mpg on a short trip with a cold engine seems reasonable. Hook a ScanGauge up to your Prius & you'll see instantaneous FE from 0 to infinity.
     
    #7 JimN, Mar 6, 2011
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