Difference in MFD MPG

Plastic tank lining bladder. If it was cold at the time of first fill and hot at last fill the bladder being more flexible when hot will hold more petrol than when cold and stiff giving a false calculated milage. The MFD is normally pretty accurate.

 

<div class='quotetop'>QUOTE(Alexstarfire @ Oct 4 2007, 11:16 AM) [snapback]521170[/snapback]</div>

It's not different. It's the same old story: the tank bladder keeps you from doing any sort of accurate fuel calculation based on a small number of fill-ups. It can't be done.

Tom

 

It's interesting that, after reading so many articles on this, people still seem to miss the salient point. Pat and Tom covered it quite nicely: you can never know how much gas you burned by looking at how much gas it takes to fill up the tank. The gas tank has different effective capacities at different times, so it takes different amounts of gas to fill up the tank.

So, when you fill up the tank, drive for a while, and then put in 10 gallons, you can not assume that you burned 10 gallons during your driving. You may have burned less than that or you may have burned more than that; you don't know!

This issue has nothing to do driving style or speed or to the makeup of your gas (i.e. "winter" vs. "summer" gas). The only important issue is the variability of the effective gas tank size (which is a combination of the flexible bladder, the sensitivity of the pump shutoff mechanism, and how aggressively you "top off").

 

Without effort, it's impossible to accurately fill the tank. With effort, you tick-off the folks who gripe about every drop of fuel that fails to stay in the tank.

If your calculations are off by 10% or more, like you added 11 gallons when only 10 should have fit, then start worrying...

 

I've been keeping track since tank 1 and have found the MFD to be more accurate than I would expect. I calculate the number of gallons that were used based on the MFD and subtract that from my actual pumped gallons. My max difference was a gallon in either direction. My spreadsheet is at work but my memory is that in 6000 miles my difference between gallons pumped and gallons used is about 2.5 gallons. My average MPG is 52 so I've used about 115 gallons. That's about 2.2%.

Temperature variations and pump accuracies can count for most of this total difference. Next I'd have to start considering all sorts of things I learned in college like partial pressures and how much gas I'm loosing through evaporation each time I remove the gas cap to fill up and all the vapor lost while filling. (How much gasoline is in the 9 gallons of vapor forced out per fill-up?)

I don't care what kind of car you drive and how it's designed expecting accuracies better than this is unrealistic.

Anyone know the accuracy that Toyota claims to achieve in it's MFD? I'm truly curreous.

 

I've been keeping track of my 'calculated' MPG and my MFD-displayed MPG by tank for the 10,000 miles I've owned my Prius. I have cumulative weighted-average calculations for both sets of MPG numbers, and they've stayed pretty consistent with the calculated weighted-average MPG being about 1 MPG less than the weighted-average MFD-displayed MPG. So that's roughly a 2% difference.

(I also have these comparisons based on each of the last 5 tanks, also consistent with my 'cumulative' numbers.)

Most of the time, on individual tanks, my calculated MPG is 1 - 2 miles less than the displayed MPG, but, due to the 'bladder effect' and the fact that here in New Jersey we don't pump our own gas, I've had individual tank measurements as much as 7 MPG less than the displayed and as much as 4 MPG more than the displayed.