New Prius - Large MFD-Pump discrepancy

colden · 07-11-2007, 10:04 PM

We just got our new 07 prius a couple of weeks ago. This forum was helpful in a lot of ways, including in learning how to maxamize gas mileage. Thanks to everyone.

Anyway, the MFD said we averaged 47.6 MPG over our first 479 miles. We went to fill up our first tank, and we only put 6.728 gallons in it. The odometer itself actually reads 490 miles so that comes out to 72.83 MPG. Why the huge discrepancy?

The tank is definitely full. The auto shut-off from the pump came on more than once. The gauge in the prius shows completely full as well.

What are we missing here? Thanks in advance.

P.S. I guess the answer is the bladder. But is this big of a discrepancy something to be worried about? Or is it just because it is the first tank? Thanks.

berylrb · 07-11-2007, 10:31 PM

<div class='quotetop'>QUOTE(colden @ Jul 11 2007, 09:04 PM) [snapback]477253[/snapback]</div>

Quote:

We just got our new 07 prius a couple of weeks ago. This forum was helpful in a lot of ways, including in learning how to maxamize gas mileage. Thanks to everyone.

Anyway, the MFD said we averaged 47.6 MPG over our first 479 miles. We went to fill up our first tank, and we only put 6.728 gallons in it. The odometer itself actually reads 490 miles so that comes out to 72.83 MPG. Why the huge discrepancy?

The tank is definitely full. The auto shut-off from the pump came on more than once. The gauge in the prius shows completely full as well.

What are we missing here? Thanks in advance.

P.S. I guess the answer is the bladder. But is this big of a discrepancy something to be worried about? Or is it just because it is the first tank? Thanks.
[/b]

Does the odometer read 490 or the "A" setting (I'm assuming that 2007 still has the odometer, A and B options)? This can account for 11 miles of driving around the dealership parking lot, but you said 479 so I could just as easily assume that 479 is the actual miles you drove from the purchase until today, that is the "A" mileage for this tank

Either way, assuming the MFD is correct 47.6 MPG, then you are right either the bladder fluctuation accounts for some of the "extra" 3.27 gallons that seem to have disappeared or they were somehow consumed prior to your obtaining possession of the car, WoW, that's a lot of gas for 11 miles! I don't know if you can burn that much just sitting with the engine on in the dealer parking lot, showing buyers how the buttons work, wash area and putting license plates on, wow! So most likely you're correct bLaDdER fluctuations and/or idle time at the dealer.

Oh yeah, welcome to PC.com

cheers

dogfriend · 07-11-2007, 11:56 PM

<div class='quotetop'>QUOTE(colden @ Jul 11 2007, 07:04 PM) [snapback]477253[/snapback]</div>

Quote:

We just got our new 07 prius a couple of weeks ago. This forum was helpful in a lot of ways, including in learning how to maxamize gas mileage. Thanks to everyone.

Anyway, the MFD said we averaged 47.6 MPG over our first 479 miles. We went to fill up our first tank, and we only put 6.728 gallons in it. The odometer itself actually reads 490 miles so that comes out to 72.83 MPG. Why the huge discrepancy?

The tank is definitely full. The auto shut-off from the pump came on more than once. The gauge in the prius shows completely full as well.

What are we missing here? Thanks in advance.

P.S. I guess the answer is the bladder. But is this big of a discrepancy something to be worried about? Or is it just because it is the first tank? Thanks.
[/b]

Different pumps have a different sensitivity for auto shut off. So the dealer's pump may have gotten the tank "fuller" than the pump you used. I have found that individual tanks may vary wildly compared to the reading on the MFD, but if you average 3 or more fillups, the numbers agree more or less.

Also, be careful topping up, because it is possible to damage the evaporative emissions system by getting liquid gas into the charcoal canister or between the outer tank and the bladder. Most people seem to agree that you should pump on the slowest setting and then stop after it clicks off.

qbee42 · 07-12-2007, 09:11 AM

It's normal. The bladder will cause a lot of variation from pump to pump, and fill to fill. Only long term statistics will work for manual calculations, as the variation will cancel out. The MFD does a pretty good job.

As others have stated, be careful topping off. I don't do it.

Tom

richard schumacher · 07-12-2007, 09:22 AM

Yes, the fuel tank bladder is variable. This means that the amount of fuel you put in at this fillup is not equal to the amount that you burned since the last fillup. See the difference? In your example, you certainly did drive 479 miles (because the odometer is always accurate), but you certainly did not burn 6.728 gallons while you drove those 479 miles. Instead you burned something like 10.1 gallons (= 479/47.6). This effect occurs in all cars; the variable capacity bladder greatly magnifies it.

The important thing to remember is that the "you're almost out of gas" warning (the last pip on the guess gauge starts to flash) *always* means "buy gas now, dammit". You might be able to drive 40 miles on what's still in the tank, or you might be able to drive one mile. There's no way to know. The websites are filled with sad tales of people who were certain that they could drive another 35 miles based on the distance since their last fillup, and were wrong.

patsparks · 07-12-2007, 09:49 AM

I always try to fill at the same pump at the same location. I would never try to calculate economy if I didn't put both first and second fill in the tank. Or if I did I wouldn't trust it.

priusmaybe · 07-12-2007, 01:00 PM

I was down to one pip (non flashing) for a while. I was afraid I would run out of gas, so I stopped at the first station I saw (shell). The pump cut off at 7 gallons. I am in hot humid Atlanta. When I got back in the car it registered completely full

I wish the tank/guage were more accurate. How could one pip used indicate 3 gallons of gas?

FL_Prius_Driver · 07-12-2007, 10:01 PM

I have a '07 and the first fillup was very cranky. Wanted to trip at 5 gallons. Had to force feed to reasonable amount (~8.5 gallons). Following fillups went much better, but there are a great many pumps set to trip prematurely when filling a Prius.

FinnAgain · 07-13-2007, 10:42 AM

Same kind of story here, and I am hoping to get some insight - because I am very confused.

I have filled up four times since buying my '07 Prius. Here is the breakdown for mileage:

car's screen says:****manual calc:

48.8***************64.38
50.0***************43.20
50.3***************56.47
50.2***************40.36

I live in South Florida. I have not altered the climate settings much at all (on auto the entire time).
I'm certain my manual calculations are accurate (I'm a fanatic about acuracy - just ask my wife). My driving conditions are extremely consistent - 85-90% hwy, same route through the city to and from work. If I take the "word" of the car's calculations, everything seems fairly well. I mean, according to it, the mileage dips when I go too fast on the hwy, and rises when I drive around town (and have greater control over usage). But that would mean that the bladder has varied by something like 35% so far.

And here's the most amazing part. This morning when I got out of the car, there were 178 miles on this tank of gas and the fuel guage still has all 10 bars. I have a hypothesis on this. I think the wide fluctuations are the result of different gas pumps. The hi-lo pattern that has begun leads me to believe this. And this morning's observation supports the idea. And I think the real culprit is the bladder - not the pump. I have calculated mileage on every tank since I was 16 (I'm 32), and there have never been such huge variables on any of the previous 4 vehicles I have owned. The only variable here is the bladder, and I think the pumps are not shutting off correctly because of it.

Questions: How does the onboard computer determine mileage? How does the fuel guage determine available fuel? How might the gas bladder be undermining my calculations? What can I do to prevent its effect? Have I missed a variable that may be thwarting me?

christob · 07-13-2007, 11:04 AM

It is also interesting to see, Finn, that your 4 calc'd values average 51.1 mpg.
I've pretty much given up on "caring" about the per-tank MPG, if for no other reason that the inability to put the same amount of gas into the bladder at each fillup! (And I haven't hit a winter yet, when the bladder will likely be even less likely to expand.) Being on 1 bar and the tank taking 10gals once, and 8 gals the next time, is just too much variation to try and deal with.
I consistently see the MFD showing me 50.1 - 50.4 values by the time I fill up... I do keep a log in Excel, and my per-tank calc'd MPG values are always a bit different from the MFD (though not yet as wildly different as yours.) So I've decided to just watch the cumulative, ownership-to-date MPG; ie, the sum of all my gallons filled, when I pull into the gas station "empty", vs. the total miles driven at that point. I figure the bladder disparities will average out over time & tanks...
So far, that calculated to-date value hovers right around 51-52 mpg, which is "close enough" to the 50.1 value I usually see on the MFD.

07-12-2007, 09:22 AM	#5
richard schumacher stygian pigeon Join Date: Mar 2004 Location: United States Posts: 4,232 My Car: 2004 Prius Package: #6 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Yes, the fuel tank bladder is variable. This means that the amount of fuel you put in at this fillup is not equal to the amount that you burned since the last fillup. See the difference? In your example, you certainly did drive 479 miles (because the odometer is always accurate), but you certainly did not burn 6.728 gallons while you drove those 479 miles. Instead you burned something like 10.1 gallons (= 479/47.6). This effect occurs in all cars; the variable capacity bladder greatly magnifies it. The important thing to remember is that the "you're almost out of gas" warning (the last pip on the guess gauge starts to flash) always means "buy gas now, dammit". You might be able to drive 40 miles on what's still in the tank, or you might be able to drive one mile. There's no way to know. The websites are filled with sad tales of people who were certain that they could drive another 35 miles based on the distance since their last fillup, and were wrong. __________________ Copyright (C) 2009. All rights reserved.

07-13-2007, 11:04 AM	#10
christob Join Date: Jun 2007 Posts: 175 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	It is also interesting to see, Finn, that your 4 calc'd values average 51.1 mpg. I've pretty much given up on "caring" about the per-tank MPG, if for no other reason that the inability to put the same amount of gas into the bladder at each fillup! (And I haven't hit a winter yet, when the bladder will likely be even less likely to expand.) Being on 1 bar and the tank taking 10gals once, and 8 gals the next time, is just too much variation to try and deal with. I consistently see the MFD showing me 50.1 - 50.4 values by the time I fill up... I do keep a log in Excel, and my per-tank calc'd MPG values are always a bit different from the MFD (though not yet as wildly different as yours.) So I've decided to just watch the cumulative, ownership-to-date MPG; ie, the sum of all my gallons filled, when I pull into the gas station "empty", vs. the total miles driven at that point. I figure the bladder disparities will average out over time & tanks... So far, that calculated to-date value hovers right around 51-52 mpg, which is "close enough" to the 50.1 value I usually see on the MFD. __________________ 6/13/07 Delivery of my Silver '07 pkg2 Disabled reverse beeper, clear door-edge guards, ground-loop noise filter for AUX jack ...Wishlist... Toyota-produced iPod interface--until then, DICE or VAIS, Tint, Door moldings, Sunroof Consistently 52-54mpg on MFD, cumulative calc'd at 52.8

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07-11-2007, 10:04 PM	#1
colden Join Date: Jul 2007 Posts: 1 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	We just got our new 07 prius a couple of weeks ago. This forum was helpful in a lot of ways, including in learning how to maxamize gas mileage. Thanks to everyone. Anyway, the MFD said we averaged 47.6 MPG over our first 479 miles. We went to fill up our first tank, and we only put 6.728 gallons in it. The odometer itself actually reads 490 miles so that comes out to 72.83 MPG. Why the huge discrepancy? The tank is definitely full. The auto shut-off from the pump came on more than once. The gauge in the prius shows completely full as well. What are we missing here? Thanks in advance. P.S. I guess the answer is the bladder. But is this big of a discrepancy something to be worried about? Or is it just because it is the first tank? Thanks.

07-12-2007, 09:11 AM	#4
qbee42 Senior Member Join Date: Mar 2006 Location: Northern Michigan Posts: 6,236 My Car: 2006 Prius Package: #7 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 6	It's normal. The bladder will cause a lot of variation from pump to pump, and fill to fill. Only long term statistics will work for manual calculations, as the variation will cancel out. The MFD does a pretty good job. As others have stated, be careful topping off. I don't do it. Tom

07-12-2007, 09:49 AM	#6
patsparks Uneducated bird-brain Aussie Join Date: Jul 2007 Location: Adelaide South Australia Posts: 5,853 My Car: 2004 Prius Package: Base Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 23	I always try to fill at the same pump at the same location. I would never try to calculate economy if I didn't put both first and second fill in the tank. Or if I did I wouldn't trust it.

07-12-2007, 01:00 PM	#7
priusmaybe Senior Member Join Date: Apr 2007 Location: Atlanta Posts: 305 My Car: 2007 Prius Package: #3 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I was down to one pip (non flashing) for a while. I was afraid I would run out of gas, so I stopped at the first station I saw (shell). The pump cut off at 7 gallons. I am in hot humid Atlanta. When I got back in the car it registered completely full I wish the tank/guage were more accurate. How could one pip used indicate 3 gallons of gas?

07-12-2007, 10:01 PM	#8
FL_Prius_Driver Senior Member Join Date: Jun 2007 Location: Tampa Bay Posts: 1,369 My Car: 2001 Prius Package: N/A Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I have a '07 and the first fillup was very cranky. Wanted to trip at 5 gallons. Had to force feed to reasonable amount (~8.5 gallons). Following fillups went much better, but there are a great many pumps set to trip prematurely when filling a Prius.

07-13-2007, 10:42 AM	#9
FinnAgain Join Date: Jul 2007 Posts: 3 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Same kind of story here, and I am hoping to get some insight - because I am very confused. I have filled up four times since buying my '07 Prius. Here is the breakdown for mileage: car's screen says:**manual calc: 48.8***********64.38 50.0***********43.20 50.3***********56.47 50.2*************40.36 I live in South Florida. I have not altered the climate settings much at all (on auto the entire time). I'm certain my manual calculations are accurate (I'm a fanatic about acuracy - just ask my wife). My driving conditions are extremely consistent - 85-90% hwy, same route through the city to and from work. If I take the "word" of the car's calculations, everything seems fairly well. I mean, according to it, the mileage dips when I go too fast on the hwy, and rises when I drive around town (and have greater control over usage). But that would mean that the bladder has varied by something like 35% so far. And here's the most amazing part. This morning when I got out of the car, there were 178 miles on this tank of gas and the fuel guage still has all 10 bars. I have a hypothesis on this. I think the wide fluctuations are the result of different gas pumps. The hi-lo pattern that has begun leads me to believe this. And this morning's observation supports the idea. And I think the real culprit is the bladder - not the pump. I have calculated mileage on every tank since I was 16 (I'm 32), and there have never been such huge variables on any of the previous 4 vehicles I have owned. The only variable here is the bladder, and I think the pumps are not shutting off correctly because of it. Questions: How does the onboard computer determine mileage? How does the fuel guage determine available fuel? How might the gas bladder be undermining my calculations? What can I do to prevent its effect? Have I missed a variable that may be thwarting me?