New Prius - Large MFD-Pump discrepancy

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

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

 

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

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.

 

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

 

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.

 

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.

 

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?

 

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.

 

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?

 

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.

 

Yep, Finn Again it is the bladder. But consider the things that effect that bladder.

• temperature, I'm in LA now went from 55F to 88F, fill up increased by 3 gallons same pip,
• fill rate, that is, set pump on the same "click" I try to use the first setting, but sometimes if station tank is empty the first setting takes forever, and
• use same gas station and pump at that gas station as much as possible, different stations have different blends which effect fuel efficiency, also this means one may need to fill up at 3 pips instead of one blinking pip.

 

All in all, given that it's impossible to know that when the pump clicks off when refueling it means the same level of fuel is in the tank as the last time, I can't see much point in expecting tank-to-tank calculations to render anything approaching an assured level of accuracy. Over time and multiple fillups, discrepancies will tend to iron out, but only over time and multiple fillups.

As an example, my own Prius has shown tank-to-tank mpg calculations of 48,61,48,54 and 50, while the MFD has shown 52, 54, 54, 54, and 54. After 5 refueling stops, the MFD shows 54.00 and the calculation 53.38.

Personally, I'd broadly assume the Prius knows what it's doing when it calculates MPG, and that as a consequence the MFD is likely to be pretty close to right, and would tend to believe that more than the calculations if there's a discrepancy. After all, the MFG uses consistent and known quantities to calculate, where we don't!

 

I've found the MFD is way more consistant than calculated numbers. When my car was new I could never get more than 7 or 8 gallons in a fill and , then after about 5000 miles I had a fill where it appeared the bladder finally stretched out to allow a 10.5 gallon fill. If you average things out over time you can figure out how accurate the MFD really is. On my first Prius it was about 4% optomistic, on my 2007 its about 0.5% optimistic.

A believe the MFD calculates fuel usage based on the #1 fuel injector duty cycle. That should be pretty accurate, although over the long haul it may change a little as the fuel injector ages.

 

Thank you very much. I have been befuzzled about it. Of course, if I am understanding the circumstances properly, I we all have what amounts to a variable-sized fuel tank. This leads me to some other questions:

1. How reliable is the gas guage at displaying remaining gas?
2. Should I be concerned about running low/out of gas when the guage claims otherwise?

 

<div class='quotetop'>QUOTE(FinnAgain @ Jul 13 2007, 11:53 AM) [snapback]478141[/snapback]</div>

There are those here who will refer to it as the 'guess gauge', but it's really not quite THAT bad. I've found it's behavior is pretty consistent from fill up to fill up, in that it lingers on full for around 170 miles, then drops the next two blobs each at about 30 miles, then the next few at about 50 miles apiece. It's really no worse than any other fuel gauge.

I take the view that once the gauge indicates one solid blob, it's time to refuel, even though that blob may last another 50 miles before it starts blinking and refueling becomes critical. However, to some extent each car is different and with each driver getting differing fuel efficiency, the experience of one person may not be a directly reliable guide for another.

 

<div class='quotetop'>QUOTE(ZA_Andy @ Jul 13 2007, 12:16 PM) [snapback]478161[/snapback]</div>

on this tank I did not stay on full as long - I went down a blip after 44 miles, so I guess the tank was not full.
It is suppossed to be in the 90's and I will fill up again.

 

I've had my Prius for a little over 2 weeks, and I'm about ready to make my first fill-up. I've been following advice here for mileage calculations, and that's been helpful. I'm still not sure about resetting the MFD after fill-up. My understanding is that resetting it will give you tank-by-tank MPG, and leaving it alone will give you averaged lifetime MPG. Which is more useful? I would think tank-by-tank, since you could factor in weather/temperature/other circumstances and would gain a better understanding of your driving habits. Overall MGP would tend to stay very constant over a long term, and be less informative, wouldn't it?

 

When I get down to 2 pips, it's time to fill up Rudy.

Perhaps I'm a child of my upbringing, as it were... I spent 25 winters in Rural Minnesota, and letting your car get below a half-tank could be a 'death sentence', should you get stuck somewhere! I get a little uncomfortable when my tank is under a half... a quarter is anxiety time... 1 pip would probably put me in a panic!

I use a journal for tracking mpg... and average out when I get more than 5 tanks. I'm putting my 'lifetime' mpg average in my signature now.