MPG - mfd vs spreadsheet

The real MPGs are very nearly what's shown on the MFD. The system counts actuations of the fuel injectors, and so, unlike the manual method, does not depend on knowing exactly how much fuel is (or was) in the gas tank. The manual method applied to *one* fill-up can be very inaccurate in a Prius because there is no guarantee that the amount of gas you put in on this fillup is equal to the amount of gas burnt since the last fillup.

 

How many tanks are you figuring your spreadsheet on? You can't draw conclusions from, say, just 2 or 3 tanks. The calculated number will become progressively more accurate with more tanks.

Also, is your spreadsheet simply averaging MPG for each fillup or does it consider miles driven? In my spreadsheet I have a calculated column for "weighted difference", using this formula:

=(F2*(C2/AVERAGE(C:C)))

... where column F is the difference between displayed and calculated MPG and column C is the miles per tank. I then average this calculated column for my cumulative difference.

 

I just filled my third tank, and estimated how many gallons I should get based on dividing the MFD mpg into the miles driven on that tank. I came very close to that figure (8 gallons) when the pump clicked off. Added a soupcon of petrol lagniappe and off I went. The MFD and my hand calculator were just over 2 mpg apart. I check carefully the point at which the first pip comes off, so over time that will give me a pretty good idea of how many miles I'll get on that tank.

 

That would work, assuming that the bladder would let you add that much. (Yes, it *seems* as though it should, but it may not always be possible.)

But is it worth the bother? For a single tank, trust the MFD. For a long-term figure, the inaccuracy inherent in each individual fillup averages to zero and quickly becomes insignificant, so it's sufficient just to take the total miles and the total of the gas pump readings.

Keep in mind that fuel pumps are not perfectly accurate either. By some strange coincidence they seem usually to be inaccurate in favor of the gas station owner, which makes manually-computed MPGs smaller than MFD MPGs.

 

Hopefully you now have had a couple more tanks of gas and can see there is variation in the amount of gasoline you can add to your car.

Warning: Do not rely on the MFD to tell you how much gasoline you "should" be able to put in the tank. Take it to the automatic fill and one click beyond, and you should be OK. Even if you "know" the tank "should" take more, DON'T. Do a search on PriusChat to see the downside of overfilling the tank (and bladder).

Also, do a search on MFD vs. MPG, and you'll see several long threads (some of which are very good) in which the differences and fluctuations are well-discussed and explained.

Enjoy your new Prius!!

 

So should we still hit the reset button after refill.... (I haven't even filled my car yet?)...

 

50 tank refills spreadsheet calculation: 50.72 mpg

MFD.............................................. 51.6 mpg

07 Prius

 

Not bad. I assume the spreadsheet fuel data comes from the pump readings? For those doing manual calculations, keep in mind that gas pumps are not 100% accurate. Typically they are allowed some set amount of error. You can guess which way the error usually falls.

Tom

 

Yes, the spreadsheet fuel data was from the pump readings.

 

It depends on what you want to see. If you want fresh MPG figures as you drive from that point, then yes. If you want the MFD to report net MPGs since the last time it was reset, then no.

 

Hmm, not sure what you've done with that last column.

Let's assume you have columns A through G there and your headings are in row 1. Replace what you have in cell G2 and below with a formula that figures the difference between columns E and F:

[EDIT/CORRECTION]
A big brain f**t here. Forget this:

=F2-E2

and use this, formatted as a percentage:

=F2-E2/E2

[EDIT COMPLETE :blush::der:]

Now in cell H2 and below use my formula but change the reference to column F (where I figure the difference between the two methods) to column G:

=(G2*(C2/AVERAGE(C:C)))

Format that as a percentage. That gives a "weighted difference" for each tank, weighted by how many miles were driven. Finally in cell I2 use this, also formatted as a percentage, to figure the cumulative difference:

=AVERAGE(H:H)

I didn't take time to transpose your numbers and calculate a cumulative difference, but the tank-to-tank differences appear well within reason. Your last tank's difference is greater than the rest, but with the Prius' flexible fuel bladder you can expect that sometimes -- see my two April fillups below. That's why you can't rely on calculated MPG for just one tank; the more you have, the more accurate it becomes.

To reassure you that you're in the ballpark with your tank-to-tank differences, here is my spreadsheet beginning last October when I first started tracking mine:

 

Send me your e-mail by PM and I'll be glad to send you my file. And congrats on the results!

 

the only MPG i trust is the life time mileage divided by total purchased. 8200 miles / 164.565 gallons = 49.8 MPG.

 

I second this notion...

 

No question that's the most accurate method. But that won't help much for results from individual tanks, nor will it tell me how far off the MFD is from actual. That's why I use the spreadsheet. Now with several months' data, I trust the MFD's results to be within a 2-3% margin of error.