Computer vs. computed MPG

BrianG · 03-11-2006, 07:41 PM

Wanted to put my two cents worth in here...

For the past 6009 miles I have compared what the computer says my mpg is to what I computed it to have been.

Computer average: 46.04
Computed average: 43.23

This mileage includes weekday stop and go rush hour traffic, several 400 mile weekend trips, one 1700 mile long weekend trip and the typical weekend errands. We all know that auto manufacturers inflate the estimated mpg numbers but it is dismaying to discover the computer does as well.

BrianG
2005 Driftwood Pearl Prius
18367 mi.

aforkosh · 03-11-2006, 10:18 PM

In this thread, I posted my results over the year 2005. I found that the mileage reported on the MFD was 2.73% higher than than computed from my written fuel log. This amounted to about 8 galllons of gas over about 15000 miles.

I did come up with several possible explanations for the discrepency not all of which point to chicanery on the part of Toyoto:

1) Evaporation from the tank--not likely due to the bladder

2) Inaccuracy in measuring fuel flow from the tank to ICE, leading to a error on the MFD side

3) Commerical fuel pump not accurately recording fuel added. This would be related to how fuel was added. After auto-fill had stopped, I ususall managed to get 1.5 to 3 gallons in in small portions. I never overfilled the tank, but it is quite possible that despite certification, the gague at pumps may read high for these short hits. This year, I am deliberately not trying to max my fill, but actually shut down soon after the auto-fill stops (the top bar on the fuel gague now disappears in about 80 miles, as opposed to 120 miles before). I'll see if this makes any difference when I again do a summary.

Allannde · 03-11-2006, 10:43 PM

Quote:

Originally posted by aforkosh@Mar 11 2006, 07:18 PM
<snip>
I did come up with several possible explanations for the discrepency not all of which point to chicanery on the part of Toyota:
<snip>

There is another possibility which probably would not account for all of the difference. That is that the pump is not delivering exactly as much as it says it is delivering. There is an allowable tolerance for inaccuracy. Very few pumps are absolutely accurate. Some are very innacurrate and are the cause af many headaches. This is usually a problem only if one pumps from the same gas pump all the time and it is one of these badly inaccurate pumps.

KTPhil · 03-11-2006, 10:51 PM

And then there are the dishonest station operators who are good buddies with the weights and measures inspector... I've got one in my town. According to their pump, I put over 17 gallons in a 16 gallon tank. There is no initial on the cert sticker, except when I complained and a "surprise" (yeah, right) inspection. The initials disappered a week later. I trust Toyota more than my local stations!

DaveinOlyWA · 03-11-2006, 10:51 PM

gee. you trying to tell me my MFD is wrong despite it varying (and LOWER than my calculated figure) less than a fraction of a mpg??

that is disappointing to me. i just thought i had done a good job of using pumps that were accurate.

hdrygas · 03-11-2006, 11:33 PM

Mine has generally been a bit higher than the computer, but over time they are converging that is after a year and 3/4.

kingofgix · 03-11-2006, 11:38 PM

Quote:

Originally posted by DaveinOlyWA@Mar 11 2006, 10:51 PM
gee. you trying to tell me my MFD is wrong despite it varying (and LOWER than my calculated figure) less than a fraction of a mpg??

that is disappointing to me. i just thought i had done a good job of using pumps that were accurate.

[snapback]223716[/snapback]

for what its worth, I have meticulously tracked every gas purchase since I puchased my Prius, and have kept track of every useful piece of info in an Excel spreadsheet.

Hear is what I have found:
Miles driven - 29,230
Gallons gas purchased - 572.68
Actual lifetime mpg - 51.04

WEIGHTED average of all MFD readings - 51.40

Virtually identical. I would ask, are you sure you are comparing actual lifetime mpg to a WEIGHTED average of all MFD readings? If you don't weight the mfd readings based on the # miles driven, the comparison is meaningless.

DaveinOlyWA · 03-12-2006, 12:11 AM

Quote:

Originally posted by kingofgix@Mar 11 2006, 08:38 PM
for what its worth, I have meticulously tracked every gas purchase since I puchased my Prius, and have kept track of every useful piece of info in an Excel spreadsheet.

Hear is what I have found:
Miles driven - 29,230
Gallons gas purchased - 572.68
Actual lifetime mpg - 51.04

WEIGHTED average of all MFD readings - 51.40

Virtually identical. I would ask, are you sure you are comparing actual lifetime mpg to a WEIGHTED average of all MFD readings? If you don't weight the mfd readings based on the # miles driven, the comparison is meaningless.

[snapback]223731[/snapback]

i calculate LT mileage by determining the amount of gas used for each tank both calculated and based on the pump reading.
iow, if i went 500 miles and the MFD says 50 mpg, then i calculate what the MFD says i should have used, using tank mileage (for this i use trip A to get a more accurate reading in tenth mile increments. both trip A and MFD are always reset manually by me at the same time always when the car is at rest) divided by mpg's to determine calculated gallons of gas used.

using the method based on the MFD, my car has burned 423.798 gallons of gas. according to the gas pump, i have put in 423.173 gallons of gas. if you read my signature, you will know that the above difference equates to 1/16th of a mile per gallon or .06 mpg * 423.173 gallons or about 25 miles.

ya... i guess if you look at it that way... 25 miles is pretty inaccurate after all. i am bummed now

jeneric · 03-12-2006, 03:28 AM

Quote:

Originally posted by DaveinOlyWA@Mar 11 2006, 09:11 PM
using the method based on the MFD, my car has burned 423.798 gallons of gas. according to the gas pump, i have put in 423.173 gallons of gas. if you read my signature, you will know that the above difference equates to 1/16th of a mile per gallon or .06 mpg * 423.173 gallons or about 25 miles.

ya... i guess if you look at it that way... 25 miles is pretty inaccurate after all. i am bummed now

[snapback]223742[/snapback]

I'm curious if you're in the rounding error range. So when your MFD says 51.4 mpg, I take that to mean from 51.350mpg to 51.44999999...mpg. And the same thing for miles. So, if you add .049 to each mpg calculation and subtract .05 for each mile reading (although the miles wouldn't get reset and would seem to mostly even out over time), how's it look? What about the other way around?

Walker1 · 03-12-2006, 09:03 AM

Both my wife and I say as long as we get 50 MPG or more we're happy campers! I find that my manual method is higher than the computer more than not. I have NO complaints. Saving at least $20.00/wk. on gas. That's almost what my interest on the loan is this month. (I hate to pay a bank any interest!).

03-11-2006, 10:18 PM	#2
aforkosh Senior Member Join Date: Jan 2004 Location: Oakland, CA Posts: 121 My Car: 2004 Prius Package: #9 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	In this thread, I posted my results over the year 2005. I found that the mileage reported on the MFD was 2.73% higher than than computed from my written fuel log. This amounted to about 8 galllons of gas over about 15000 miles. I did come up with several possible explanations for the discrepency not all of which point to chicanery on the part of Toyoto: 1) Evaporation from the tank--not likely due to the bladder 2) Inaccuracy in measuring fuel flow from the tank to ICE, leading to a error on the MFD side 3) Commerical fuel pump not accurately recording fuel added. This would be related to how fuel was added. After auto-fill had stopped, I ususall managed to get 1.5 to 3 gallons in in small portions. I never overfilled the tank, but it is quite possible that despite certification, the gague at pumps may read high for these short hits. This year, I am deliberately not trying to max my fill, but actually shut down soon after the auto-fill stops (the top bar on the fuel gague now disappears in about 80 miles, as opposed to 120 miles before). I'll see if this makes any difference when I again do a summary. __________________ Alan Forkosh Tideland Pearl #9--2/9/2004 Plate: AL4KOSH

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03-11-2006, 07:41 PM	#1
BrianG Junior Member Join Date: Mar 2005 Location: Southeastern North Carolina Posts: 18 My Car: 2005 Prius Package: #5 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Wanted to put my two cents worth in here... For the past 6009 miles I have compared what the computer says my mpg is to what I computed it to have been. Computer average: 46.04 Computed average: 43.23 This mileage includes weekday stop and go rush hour traffic, several 400 mile weekend trips, one 1700 mile long weekend trip and the typical weekend errands. We all know that auto manufacturers inflate the estimated mpg numbers but it is dismaying to discover the computer does as well. BrianG 2005 Driftwood Pearl Prius 18367 mi.

03-11-2006, 10:51 PM	#4
KTPhil Senior Member Join Date: Jan 2005 Posts: 1,213 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	And then there are the dishonest station operators who are good buddies with the weights and measures inspector... I've got one in my town. According to their pump, I put over 17 gallons in a 16 gallon tank. There is no initial on the cert sticker, except when I complained and a "surprise" (yeah, right) inspection. The initials disappered a week later. I trust Toyota more than my local stations!

03-11-2006, 10:51 PM	#5
DaveinOlyWA Opps !! I Did it Again!! Join Date: Apr 2004 Location: South Puget Sound, WA Posts: 9,521 My Car: 2006 Prius Package: #4 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 4	gee. you trying to tell me my MFD is wrong despite it varying (and LOWER than my calculated figure) less than a fraction of a mpg?? that is disappointing to me. i just thought i had done a good job of using pumps that were accurate.

03-11-2006, 11:33 PM	#6
hdrygas Senior Member Join Date: May 2004 Location: Olympia Wa Posts: 3,650 My Car: 2004 Prius Package: #9 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Mine has generally been a bit higher than the computer, but over time they are converging that is after a year and 3/4.

03-12-2006, 09:03 AM	#10
Walker1 Senior Member Join Date: Jan 2006 Location: FL Posts: 881 My Car: 2008 Prius Package: #2 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Both my wife and I say as long as we get 50 MPG or more we're happy campers! I find that my manual method is higher than the computer more than not. I have NO complaints. Saving at least $20.00/wk. on gas. That's almost what my interest on the loan is this month. (I hate to pay a bank any interest!).