MPG - mfd vs spreadsheet

[sleonardelli]

I have been tracking my 08 Touring mpg via spreadsheet since April. My question is actually in reference to the fuel bladder and mpg because I wonder if I'm filling my Pri enough to get an accurate measure of MPG on a spreadsheet. Today, for example, I filled at the same Sam's club I always do and filled 1 click (as I always do) beyond the auto-click. My MFD read 42.8 mpg but my spreadsheet indicates 48.2. Granted the spreadsheet doesn't always indicate better MPG. Considering the fuel bladder variables, how do I really know what my MPG is?

[richard schumacher]

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.

[sleonardelli]

So, if my MFD indicates I need 5.7 gallons (I guess I'll need to carry a calculator) I should have filled to that point instead of the one extra click?

[JimboK]

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.

[Fraser]

Originally Posted by sleonardelli

So, if my MFD indicates I need 5.7 gallons (I guess I'll need to carry a calculator) I should have filled to that point instead of the one extra click?

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.

[richard schumacher]

Originally Posted by sleonardelli

So, if my MFD indicates I need 5.7 gallons (I guess I'll need to carry a calculator) I should have filled to that point instead of the one extra click?

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.

[a priori]

Originally Posted by sleonardelli

So, if my MFD indicates I need 5.7 gallons (I guess I'll need to carry a calculator) I should have filled to that point instead of the one extra click?

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!!

[sorthund]

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

[Cacti]

50 tank refills spreadsheet calculation: 50.72 mpg

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

07 Prius

[qbee42]

Originally Posted by Cacti

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