Actual MPG vs. MFD reading Comparison Spreadsheet

[Kenko74]

After 3200 miles, I reluctantly have to agree that the MFD display MPG is close to reality. The reason I say reluctantly, is that on a tank-to-tank basis, my actual MPG has never agreed with the MFD value (which I reset after every fill up), sometimes being as high as 14 MPG different. But I've kept detailed records of my actual gasoline purchases/MPG with the MFD readings, and after 10 refillings, the average difference between the actual MPG and the MFD readings is only 0.2 MPG.

I've created a nifty spreadsheet that you can use to compare all this. Just fill in the blanks, it does all the calculations. You can, of course, delete my last ten tankfulls of data. I just left them in there so you can see how I justified that the MFD is accurate.

[fred garvin]

Originally Posted by Kenko74

After 3200 miles, I reluctantly have to agree that the MFD display MPG is close to reality. The reason I say reluctantly, is that on a tank-to-tank basis, my actual MPG has never agreed with the MFD value (which I reset after every fill up), sometimes being as high as 14 MPG different. But I've kept detailed records of my actual gasoline purchases/MPG with the MFD readings, and after 10 refillings, the average difference between the actual MPG and the MFD readings is only 0.2 MPG.

I've created a nifty spreadsheet that you can use to compare all this. Just fill in the blanks, it does all the calculations. You can, of course, delete my last ten tankfulls of data. I just left them in there so you can see how I justified that the MFD is accurate.

that is very nice.

2 observations:

1 - i see you got better over time
2 - i wonder if the variability of the delta will be smaller in the bladerless 2010?

[qbee42]

Originally Posted by fred garvin

that is very nice.

2 observations:

1 - i see you got better over time
2 - i wonder if the variability of the delta will be smaller in the bladerless 2010?

#2 - Without a doubt. There is still some variation with gas pumps and filling techniques, but the bladder is the biggest wild card.

Tom

[JimboK]

Nice job. Thanks on behalf of those who don't already have such a tool.

One small flaw: with the tank-to-tank variability in miles driven, your averages in columns K & L will be off a bit. For example, the 421 mile tank in row 11 contributes significantly more to lifetime results than the 138 mile tank in row 10. You might consider a formula like I have in mine to adjust for the variation. The formula provides a weighted per-tank MPG difference based on miles driven, and then the weighted differences are averaged for the lifetime number. I plugged your numbers into my spreadsheet and it calculated a lifetime 3.3% difference.

I've attached my spreadsheet, which you're welcome to steal from as you see fit. I don't track lifetime fuel economy -- I rely on CleanMPG's database for that -- but I do track fuel economy calculation differences. But the concept of the formula will work for column K as well as column L.

[Frayadjacent]

I would suspect you are correct. With the fuel bladder causing inconsistent fill volumes, your tabulated economy will likely vary widely from the MFD rating. I saw this on my last tank - it was HOT outside, and I was able to get in more gas than I had before given the state of the fuel gauge. It calculated out to only 43.xxMPG. I'm about 320 miles into the tank and it just hit 1/2. More distance than usual before that 6th bar went dark.

We really have to look at the average over a long period - months or years - to verify the MFD average. I use mymilemarker which shows a nice graph of historic economy.

[john1701a]

Originally Posted by fred garvin

i wonder if the variability of the delta will be smaller in the bladerless 2010?

There are other factors at play... and so far, they appear to amplify the guestimation error... comparing my 118,185 mile spreadsheet for the Iconic to my 2010.

Having the engine shut off more often and longer is the biggest influence, I believe.
.

[Kenko74]

Originally Posted by JimboK

Nice job. Thanks on behalf of those who don't already have such a tool.

One small flaw: with the tank-to-tank variability in miles driven, your averages in columns K & L will be off a bit. For example, the 421 mile tank in row 11 contributes significantly more to lifetime results than the 138 mile tank in row 10. You might consider a formula like I have in mine to adjust for the variation. The formula provides a weighted per-tank MPG difference based on miles driven, and then the weighted differences are averaged for the lifetime number. I plugged your numbers into my spreadsheet and it calculated a lifetime 3.3% difference.

I've attached my spreadsheet, which you're welcome to steal from as you see fit. I don't track lifetime fuel economy -- I rely on CleanMPG's database for that -- but I do track fuel economy calculation differences. But the concept of the formula will work for column K as well as column L.

Thanks!
Actually, if you look at the numbers, my "average MPG" is actually a lifetime average--total vehicle miles divided by the total gallons of gasoline used. So it should be a pretty good number as time goes by. I think my MPG increase over time is simply due to the warmer weather from March to June....

[JimboK]

Originally Posted by Kenko74

Thanks!
Actually, if you look at the numbers, my "average MPG" is actually a lifetime average--total vehicle miles divided by the total gallons of gasoline used.

Ah, yes, right you are! My apologies. I was focusing mainly on the formula for column L and didn't bother to inspect closely the formula in K. I ass-u-me-d it was written the same as L.

[PriusSport]

I always keep my gas receipts, and have never be able to make much sense of the calculated mpg. The MFD mpg I assume is the more accurate, though bladder fluctuations tend to average out over many fillups.

I don't know why the Prius has a bladder in the gas tank--there must be a thread or two on the subject--but the bladder concept was originally used in WWII fighters to reduce flammability of the gas tanks (the collapsing bladder reduced volatile fuel vapors.)

I would assume the bladder serves the same advantage in the Prius--reducing the possibility of gas ignition in a collision. Though it may be there for a different reason?

[fuzzy1]

... I would assume the bladder serves the same advantage in the Prius--reducing the possibility of gas ignition in a collision. Though it may be there for a different reason?

It is there to control evaporated emissions. A good idea whose time has not quite arrived.