Huge variance between computed mpg and MFD

[gnagel]

I realize that the MFD mileage and the calculated mpg will vary on an individual tank. But, the difference in mpg on my last fill-up was incredible.

Computed MPG = 49.3
MFD MPG = 57.6

What gives?

[berylrb]

How about amount of gas you put in? Really that can be the only real difference the miles can't vary, well I guess they could, but you'd know that because the miles driven shows on the MFD. But the computer makes a determination on the amount of gallons using _________?

that's a search for you ...

[cwerdna]

<div class='quotetop'>QUOTE(gnagel @ Jul 1 2006, 03:43 PM) [snapback]279809[/snapback]</div>

I realize that the MFD mileage and the calculated mpg will vary on an individual tank. But, the difference in mpg on my last fill-up was incredible.

Computed MPG = 49.3
MFD MPG = 57.6

What gives?
[/b]

How are you calculating your MPG? What's your average from both the computed mpg and MFD mpg over many tanks? While I have seen variance per fillup of several mpg, my lifetime average mpg by manual calculations is only ~1 mpg lower than the average of the MFD readings.

[gnagel]

<div class='quotetop'>QUOTE(cwerdna @ Jul 1 2006, 06:39 PM) [snapback]279836[/snapback]</div>

How are you calculating your MPG? What's your average from both the computed mpg and MFD mpg over many tanks? While I have seen variance per fillup of several mpg, my lifetime average mpg by manual calculations is only ~1 mpg lower than the average of the MFD readings.
[/b]

I am calculating my MPG by taking the total miles driven divided by the gallons of gas that I've pumped into the tank.

My life to date MPG per my manual computations is 55.9. The life to date MFD MPG is 57.6. So, the manually computed mileage is falling 1.7 mpg below that of the MFD on average.

I've only had my Prius for about three months and just over 3,000 miles.

[berylrb]

I'm thinking that last fill up you put in until the pump clicked and then this fill up you put in to the pump clicked and then a few more cubic inches, or something like that ...

[DaveinOlyWA]

never take any one tank seriously. the inability to control the fill because of the bladder makes single tank observations useless. take my advice, keep track of every drop of gas you put in.

[barbaram]

I've noticed the same. I'm beginning to thing that the display show the mileage since the last "reset"

My last tank was almost 53, display shows 48.4, lifetime is 48.5. I stopped hitting "reset" al long time ago... it's starting to coincide.

BTW we call the displays at my job IDU- intelligent display units... (or idiotic depending!)

[efusco]

<div class='quotetop'>QUOTE(gnagel @ Jul 1 2006, 07:03 PM) [snapback]279848[/snapback]</div>

I am calculating my MPG by taking the total miles driven divided by the gallons of gas that I've pumped into the tank.

My life to date MPG per my manual computations is 55.9. The life to date MFD MPG is 57.6. So, the manually computed mileage is falling 1.7 mpg below that of the MFD on average.

I've only had my Prius for about three months and just over 3,000 miles. [/b]

That works out to a 3% difference b/w your lifetime calculated and computed average. When you see a huge difference in a single tank it's usually b/c you used a different pump than usual and it severely underfilled or overfilled. It's possible that the MFD mileage got reset partly through, but that's obviously less likely and probably something you'd have noticed.

[gnagel]

It's possible that the MFD mileage got reset partly through, but that's obviously less likely and probably something you'd have noticed.[/b]

I'm certain that the MFD mileage was not reset incorrectly because I always set one of the odometer readings to coincide with the last fill-up. The MFD mileage reading always matches the odometer "trip" reading upon fill-up and then I set both to zero again.

[Jeannie]

I've had my car a little less than 3 months and just passed the 2000 mile mark. My "computed" mileage is usually about 2 MPG less than the MFD. I live in New Jersey where we don't pump our own gas, and so far I've filled up at different stations. So I know that the computed MPG depends on how much the tank is filled. One fill up gave me a computed MPG 2 miles HIGHER than my MFD! So I keep my 'computed' numbers in an EXCEL spreadsheet where it computes single-tank MPG, cumulative MPG, 2 tank MPG, 3 tank MPG, etc.