MFD Mileage and Calculated Mileage not even close

Fibb222 · 01-07-2007, 11:57 PM

Hi gang,

I can't figure out why my mfd and my calculated mileage are so different. Sometimes off 7%. Last fill up - off 14%!! If you can make sense of my attached spreadsheet please check it out and tell me if I'm doing something wrong.

Thanks

kdmorse · 01-08-2007, 02:07 AM

By my math, you and the MFD disagree by approximately 8.2L, it claims you've consumed 155.98L over 2881 km, and you've put in 164.158L. (Running totals are much more useful that tank by tank deltas)

Is it at all possible, possible at all, that when you started this spreadsheet, the tank was not 100% full? If, for example, on the first fill you ended up 10L or so shy of full for any reason, and started the spreadsheet in that condition, and then on this last fillup got the tank much closer to full, it would explain your numbers.

If so, as time goes on, the differences should even out. However it will take quite a while to even out as you are essentially averaging a 8L delta over X tanks.

Or... something could just be off..

-Ken

**Tideland Prius** · 01-08-2007, 03:35 AM

By and large, it seems fairly close except for the last two tanks. It could very well be our warmer temps (10°C today!). The cold weather expected for the mid-end of this week could hurt mileage.

I would say don't worry about it and average it out over several tanks. If you need proof, I can show you mine. You'll see some large discrepancies in some of my tanks.

Fibb222 · 01-08-2007, 10:52 AM

<div class='quotetop'>QUOTE(Tideland Prius @ Jan 8 2007, 12:35 AM) [snapback]372379[/snapback]</div>

Quote:

If you need proof, I can show you mine. You'll see some large discrepancies in some of my tanks.
[/b]

Yes, thank's that would be great.

<div class='quotetop'>QUOTE(kdmorse @ Jan 7 2007, 11:07 PM) [snapback]372372[/snapback]</div>

Quote:

By my math, you and the MFD disagree by approximately 8.2L, it claims you've consumed 155.98L over 2881 km, and you've put in 164.158L. (Running totals are much more useful that tank by tank deltas)

Is it at all possible, possible at all, that when you started this spreadsheet, the tank was not 100% full?
-Ken
[/b]

Sorry, where did you get the 155.98L figure.

Well I tried to fill it up on Oct 19th but the bladder might not have been full. I didn't try to top up for reasons well known to most PCers.

I should have mentioned that I press reset on the MFD at every fill up. I sure hope things even out eventually!

daveleeprius · 01-08-2007, 11:01 AM

<div class='quotetop'>QUOTE(Fibb222 @ Jan 8 2007, 10:52 AM) [snapback]372457[/snapback]</div>

Quote:

Yes, thank's that would be great.
Sorry, where did you get the 155.98L figure.

Well I tried to fill it up on Oct 19th but the bladder might not have been full. I didn't try to top up for reasons well known to most PCers.

I should have mentioned that I press reset on the MFD at every fill up. I sure hope things even out eventually!
[/b]

I like the fact that the milage displayed on the MFD is lifetime, not tank. I want to know my lifetime average. Lately my tank average has been right around 40mpg, still not bad at all for winter driving. I remember my 1991 Honda Civic Wagon would get 40mpg in the summertime and I was always thrilled, and that car was manual everything and no A/C.

Dave

**Tideland Prius** · 01-08-2007, 01:23 PM

<div class='quotetop'>QUOTE(Fibb222 @ Jan 8 2007, 07:52 AM) [snapback]372457[/snapback]</div>

Quote:

Yes, thank's that would be great.
[/b]

Here you go!
[attachmentid=6183]

Fibb222 · 01-08-2007, 01:52 PM

<div class='quotetop'>QUOTE(Tideland Prius @ Jan 8 2007, 10:23 AM) [snapback]372542[/snapback]</div>

Quote:

Here you go!
[attachmentid=6183]
[/b]

Is your "Avg FE" column the mileage readout from the MFD for just that tank? Then that makes me feel better.

Check out the red column I added to your spreadsheet.

By the way, at least your differences are sometimes positive and sometimes negative. My are all positive so far!

**Tideland Prius** · 01-08-2007, 07:40 PM

The "Avg FE" is the MFD readout for that tank, yes.

Hmm, yeah there are a few double-digit positive differences but for the most part, it's fairly close.

Your positives are fairly close except the last 3 tanks. I wouldn't worry about it. I definitely expect a bad tank this week because of the forecast of cold temperatures for us.

DaveinOlyWA · 01-16-2007, 12:08 PM

bouncing up and down is normal. when you look at my MFD verses pump mileage, there is little difference in the two. on my 06, if i discount the first tank (which was not as full as it could have been) i am within 2% despite having some tanks be more than 10% off.

the variation can be made less and i have been experimenting with that a little. in the last few tanks, i have calculated based on the MFD reading (i reset mine on every fillup as well, not interested in LT mileage, that is why i keep the spreadsheet!) how much gas should go in and if it goes over that amount, (twice in 5 fillups) i let it stop on its own. if it stops short of the figure, i fill to that level (3 times)

so far, the gap between the two are much larger than i had on my 04. and i can only attribute that to the fact that i got my 04 in early summer, my 06 can in mid fall. so thinking (or hoping!!) the numbers will start to merge as summer rolls around

Bill Merchant · 01-16-2007, 04:09 PM

The MFD MPG is based on the pulses your wheels put out (distance) divided by the time your injectors are open (hence, flow at pressure=volume), so it's pretty darn accurate. Far more accurate than miles, accurate to about two football fields, divided by a gallon, at varying temperatures, with varying energy content, from a tank with a bladder in it. Both imagine ideal tires, not found in the real world.

You'll find MFD- and calculated-MPG converge over time to within a few percent.

01-08-2007, 02:07 AM	#2
kdmorse Senior Member Join Date: Oct 2005 Location: Germantown, MD Posts: 198 My Car: 2006 Prius Model: Package: #7 Thanks: 0 Thanked 0 Times in 0 Posts Friends: 1	By my math, you and the MFD disagree by approximately 8.2L, it claims you've consumed 155.98L over 2881 km, and you've put in 164.158L. (Running totals are much more useful that tank by tank deltas) Is it at all possible, possible at all, that when you started this spreadsheet, the tank was not 100% full? If, for example, on the first fill you ended up 10L or so shy of full for any reason, and started the spreadsheet in that condition, and then on this last fillup got the tank much closer to full, it would explain your numbers. If so, as time goes on, the differences should even out. However it will take quite a while to even out as you are essentially averaging a 8L delta over X tanks. Or... something could just be off.. -Ken __________________ 2006 Black Package #7 Dark Grey Interior Headlight Flasher EV Mod

01-16-2007, 12:08 PM	#9
DaveinOlyWA 3rd Time was Solariffic!! Join Date: Apr 2004 Location: South Puget Sound, WA Posts: 12,375 My Car: 2010 Prius Model: IV Package: Solar Roof Thanks: 24 Thanked 121 Times in 103 Posts Friends: 10	bouncing up and down is normal. when you look at my MFD verses pump mileage, there is little difference in the two. on my 06, if i discount the first tank (which was not as full as it could have been) i am within 2% despite having some tanks be more than 10% off. the variation can be made less and i have been experimenting with that a little. in the last few tanks, i have calculated based on the MFD reading (i reset mine on every fillup as well, not interested in LT mileage, that is why i keep the spreadsheet!) how much gas should go in and if it goes over that amount, (twice in 5 fillups) i let it stop on its own. if it stops short of the figure, i fill to that level (3 times) so far, the gap between the two are much larger than i had on my 04. and i can only attribute that to the fact that i got my 04 in early summer, my 06 can in mid fall. so thinking (or hoping!!) the numbers will start to merge as summer rolls around __________________ My Blue 2010 : Last tank 541.9 @ 49.48 pump (56.7 MFD) 5.54 CPM, 21 MPH, Lifetime:5442.2 miles 56.5 MPG pump. (62.22 MFD) 4.72 CPM. Summer MPG 57.4 Winter: 49.5 My 2006 SPM: Last tank 376.6 miles @38.21 pump (40.8 MFD) 7.17 cpm winter mpg 49.64 summer mpg 53.41 lifetime: 42,563.5 miles 51.5 mpg pump (52.7 mpg MFD) 5.51 cpm My 2007 Zenn total "fuel cost" $166.58 on "about" 9599.7 miles. 1.74 cents per mile (granted i plug in for free at work!!) My Plate: DUALPWR (Dual Power)

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01-08-2007, 03:35 AM	#3
Tideland Prius Moderator of the North Join Date: Oct 2004 Location: Canada Posts: 19,606 My Car: 2010 Prius Model: N/A Package: Technology Package (Canada) Thanks: 227 Thanked 343 Times in 242 Posts Friends: 23	By and large, it seems fairly close except for the last two tanks. It could very well be our warmer temps (10°C today!). The cold weather expected for the mid-end of this week could hurt mileage. I would say don't worry about it and average it out over several tanks. If you need proof, I can show you mine. You'll see some large discrepancies in some of my tanks.

01-08-2007, 07:40 PM	#8
Tideland Prius Moderator of the North Join Date: Oct 2004 Location: Canada Posts: 19,606 My Car: 2010 Prius Model: N/A Package: Technology Package (Canada) Thanks: 227 Thanked 343 Times in 242 Posts Friends: 23	The "Avg FE" is the MFD readout for that tank, yes. Hmm, yeah there are a few double-digit positive differences but for the most part, it's fairly close. Your positives are fairly close except the last 3 tanks. I wouldn't worry about it. I definitely expect a bad tank this week because of the forecast of cold temperatures for us.

01-16-2007, 04:09 PM	#10
Bill Merchant absit invidia Join Date: May 2005 Location: USA \| Oregon \| Portland area \| 97004 \| Posts: 4,100 My Car: 2007 Prius Model: Package: #5 Thanks: 18 Thanked 10 Times in 8 Posts Friends: 60	The MFD MPG is based on the pulses your wheels put out (distance) divided by the time your injectors are open (hence, flow at pressure=volume), so it's pretty darn accurate. Far more accurate than miles, accurate to about two football fields, divided by a gallon, at varying temperatures, with varying energy content, from a tank with a bladder in it. Both imagine ideal tires, not found in the real world. You'll find MFD- and calculated-MPG converge over time to within a few percent.