Math Doesn't Work

Search the site for tales of the fuel gauge. All of your questions will be answered ... and then some.

 

You'll find calculating MPG per tank to be tricky at best - as getting it right requires that the fuel level be the same after each fillup, which is trickier than you might think. It's bad enough in other cars, when the average pump will click off with up to a gallon still free in the tank. But in the prius, the 'tank' (bladder) actually changes capacity with temperature and age. And the backpressure the bladder generates can cause some pumps to click off when the tank only contains 8 gallons or so.

On top of that (or more likely, because of that) - the fuel gauge is calibrated to read full at around 9 gallons (*). Anything beyond 9 is gravy, and is the reason we can often get 150 miles before the we loose the first pip.

All this causes two MPG calculation problems. You don't know how much gas is in your car. And because of that, you don't know how much gas you truly consumed over those 297 miles. You pumped 5.1 gallons. The MFD thinks you used 7.1 gallons (297/39). Given all of the above, a delta of two gallons is completely possible. For example, if the dealer pumped you all the way up to 11.9 gallons (unlikely, but not impossible), and then you casually filled up to only 9.9 gallons - all the math works out.

Most folks find that if they do the math over a larger number of miles (5 tanks worth for example), things come out a lot closer.

(Afterthought - was the MPG readout on the MFD reset when you started that tank? If not, it's calculation includes the previous tank, or more if it's never been reset).

(*) Your fuel gauge may vary.

-Ken

 

4/2 = 2. Math still works here.

I have noticed a 5-7% difference between MFD and calculated values, but I have not been through my first winter with the car.

 

Differences abound... the short of it is: Don't worry about it. between pump differences and bladder inconsistencies, you can't be sure you're always filling to the same point (in fact, i'd be willing to be that almost no fill goes to the same point as the previous one). So drive happy, and fill when the car tells you to (it's usually right).

 

<_< The MATH WORKS within the many variables in the subject equations. Consider the fuel bladder [sensitive to ambient temperature] having a direct effect on bladder capacity, variable station pump settings like how much back pressure is required to shut the valve off, perhaps even temperature of the fuel itself. My guess would be that there could well be as much as 10% swing due to these variables.

I treat my Prius just as I did its predecessor cars. I fill at my convenience, sometimes based on price or weather or where I anticipate driving. I have no concern over establishing "Bragging Rights" by seeing how far I could drive on one tank or how many gallons of fuel I can pump. I suppose that attitude is akin to not being overly concerned with what my friends or family thinks about my choice for transportation.

 

When the MFD displays the average mpg, how far back is it calculating? If it's going all the way back to the last time the reset button was pressed, you would expect that as you rack up more miles, the average mpg would stabilize, and evantually change very infrequently. However it doesn't seem like it does this. It almost seems like it is just averaging the last 30 minutes worth of driving. Anyone know what's going on there? :blink:

 

<div class='quotetop'>QUOTE(danatt @ Dec 5 2006, 10:04 AM) [snapback]357869[/snapback]</div>

I think what probably happened for the original poster is that the dealer had topped off the tank.

The 2004 and 2005 Priuses would reset both the MPG and the miles on the MFD when you filled the tank. I believe the 2006 and 2007 only resets the miles when the tank is filled. However, you have to consider that fact that the Prius won't recognize that you've filled the tank unless you add 3 or more gallons of gas.

So.... The original poster says he's had the car for a week and it's got 279 miles on it. He went to fill it up and only was able to put 5.1 gallons in, yielding 54 MPG by his calculation. But what if the dealer topped it off with 2 gallons when he bought it. This wouldn't have been enough for the Prius to detect a fillup and neither the MPG nor the miles on the MFD would be reset. But now we have 7.1 gallons, instead of 5.1 gallons consumed for that 279 miles. 279 / 7.1 equals 39.29, which is what the orignal poster said his MFD showed.

 

ya know, what i would do is this. you used about 7 gallons on your first fillup. i would put in at least that much even if i had to do the "click-click" thing. and for the topping off thing. i would find it hard to believe that these cars are shipped from a oil-strapped country full of gas. i be willing to bet there is no more than a few gallons in the car. so topping off the tank would most definitely be registered

 

<div class='quotetop'>QUOTE(DaveinOlyWA @ Dec 5 2006, 09:29 AM) [snapback]357915[/snapback]</div>

Don't top off. Period. end of story. Even in this case. The dealer fills the tanks before they give it to you, not the factory - it's possible they topped off, by either a little or a lot. It's also getting colder every day (at least up here it is), which means the bladder is shrinking. Fill up till it clicks, and let it go at that. You don't want to risk damaging the car by overfilling, or spilling gas when the bladder "burps", or having all that extra gas go back into the station's vapor recovery system, thus back into their tanks.

 

normally i would say dont top off either, but in this case, i think he used a faulty gas pump. also, dealers dont top off either. my 2006 was a good bit short of a full tank. look at my sig. my pump average and computer average is about a mile and ½ and shrinking. if i take out the first fillup, the difference is less than a tenth of a mile per gallon.

my first fillup according to computer should have used 8.159 gallons. actually put in 9.786 gallons. did my gas gauge register full while being seemingly 1½ gallons short?? yes it did. but lost the first bar at about 40 miles. (normally its over 100 miles)

since then, give or take, the difference between the two calculations is now

65.190 and 67.355 or about a ½ gallon increase in difference, but the next will swing about 2-4 tenths back to the computer average. so it will be back to nearly identical.

so in the posters case, i strongly believe that his fillup was short two gallons and i think the only way to resolve the question is to have the poster advise us of his next fillup

 

Every situation, and every dealer are different... Additionally, you need to take everything into consideration - colder weather = shrinking bladder, and he tried multiple pumps at different gas stations, so it's wasn't a faulty gas pump. I stand by the statement never top off - if you top off once, then you're just going to be off again on your mileage numbers when you fill up the next time, and the next, and the next...