6 MPG discrepancy ... ripped by the pump?

[upland_bill]

I've been tracking my mileage since I bought my used 2005 (http://www.wmrnet.com/prius.php) and have been pleased with the results and the accurracy of the onboard computer ... until today. For the second time, I did not go to get a fillup until the car notified me that I was low on gas (coincidence really, I planned to go this morning regardless). The car was reporting 50.5 mpg for the current tank, and I expected my manual calculations to be about the same. I was shocked when I pumped 9.8 gallons at my no-name gas station. I had never pumped more than 8.6 in any of my previous fillups. I was more shocked to see that this gave me a calculated MPG of 44, an all time low. So, yes it has been hot here in DC, but I think I used the A/C just as much in my previous tankful (which is essentially all the time), but even if the heat and the A/C did result in lower mileage, then why would the car be fooled into thinking I got 6 MPG more.

I think the gas station ripped me. Or. is a another example of the guass guage and bladder giving deceiving readings of mileage and gallons to fillup?

:^(

[Betelgeuse]

<div class='quotetop'>QUOTE(upland_bill @ Jul 20 2006, 11:08 AM) [snapback]289363[/snapback]</div>

I think the gas station ripped me. Or. is a another example of the guass guage and bladder giving deceiving readings of mileage and gallons to fillup?
[/b]

Almost certainly the latter. Although you filled 9.8 gallons this time, I suspect that you didn't actually burn 9.8 gallons in the mileage that you drove before the fillup. As you said, you only filled, at most, 8.6 gallons on any previous fillup. Say you burned 8.6 gallons in the mileage you drove before the fillup. That would imply an MPG of 50.1; quite close to the computer's estimate.

I'll just say that I think it's very hard for the computer to be wrong. For the "miles driven" calculation, it's using the same thing that you're using (i.e. the odometer), so it can't differ there. For the "gallons burned" calculation, it's using some measure of the fuel that goes into the injectors (to be honest, I don't totally understand the details). It's possible it's screwing up that calculation somehow, but it's much, much, much more likely that the fuel tank bladder issue is giving you the wrong idea about how much fuel is actually in the tank.

[efusco]

Just a variation in how much you can put in the tank. Some pumps shut off is slightly different than others. It'll all balance out in time.

[jtullos]

I'd trust the computer more in a case like this. For a few months recently, we had the computer reading around 56 MPG, and my at-the-pump calculation was wrong. First, it was 60 MPG, then 50 MPG, and it bounced between the two for a few fillups, always with the display showing around 56 MPG. I haven't been keeping a running lifetime average, but based on the miles driven, overall the computer was close to correct. There are quite a few factors involved in how much gas will go into the tank, and that can throw the mileage off quite a bit.

As for how the computer calculates the mileage, I'm just making an educated guess here, but it appears that it determines your instantaneous mileage based on the fuel injection rate and current speed, and then averages the mileage numbers it calculates. I'm thinking this because I've seen my mileage drop while sitting at a stop light with the gas engine off, but the electric engine running.

[mkaresh]

The time to get really concerned is when you're able to put more into the tank than it can hold.

Otherwise, most likely a matter of different pumps having different sensitivities, and thus cutting off sooner/later. My initial response was that the results with my own car are quite consistent, but then I realized that I generally fill up at the same place every time.

[daronspicher]

I'd bet it's a combo of a high MFD reading and per-fillup-descrepancy. You got as much as .5 to .8 more gallons in this time which is enough to effect 3 or 4 mpg.

Any one or two fillups doesn't make a number you can use to sell peanuts in the grandstands, but if you track the numbers over time, eventually you achieve a level where + or - .8 gallons doesn't impact the achieved mpg numbers.

I would be very surprised if you were to track mfd mpg against actual mpg over 10 tanks and find the mfd to be reporting above the numbers achieved at the pump.

My mfd has been in the 53's, 55's, 57's in the last 3 or 4 months, but my realtime at the pump average is holding pretty tight in the 52.xx mpg range. If I was really getting that 55 or 57 that is shown on the mfd, I'd see that 52.xx climbing toward 53 or 54 pretty quick.

If you're going to track a 10 tank average, start out by running the car low. Put the pump to fill up on the slow fill notch and let it click off. Reset the mfd.

Throw away that gas receipt.

Next fillup, do the same fill technique (slow notch, until click off) and keep that receipt + the next 10. On each receipt write the mfd reading and also reset the mfd before you pull away from the pump.

It's a bit anal, but it's how to check it without over/under fill throwing it off.