Math Doesn't Work

moose3720 · 11-25-2006, 08:49 AM

Ok, what's going on here....

2007 Touring Edition, one week old.

Driven (279) miles, fuel gauge was down to two (2) bars, computer says we averaged (39) mpg.

However, I could only put (5.1) gallons in the tank before the pump clicked off. Tried two different pumps at two different gas stations, same result. More specifically, the fuel gage showed FULL after only (5.1) gallons!

(54) MPG (actual ??) vs (39) mpg (computer average)

Also, fuel gage has (10) bars and the car holds (11) gallons, so am I correct to assume that (8) bars should not disappear with the first (5) gallons of gas?

Any ideas?

McShemp · 11-25-2006, 09:19 AM

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

**efusco** · 11-25-2006, 09:55 AM

It takes some getting used to and it's definately not something to start sweating over with your first fill up. Try other stations too, I find some pumps routinely cut out early at some stations and my usual station gives me the closest to a 100% fill. The bladder, the guage, and the entire fuel guage system is quite quirky and even buggy, but with time and experience you just get used to what's normal and what's not and how to get maximum distance without running out of gas.

kdmorse · 11-25-2006, 04:23 PM

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

VABeachPrius · 11-28-2006, 04:18 AM

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.

eagle33199 · 11-28-2006, 09:33 AM

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).

Charles Suitt · 11-28-2006, 01:53 PM

<_< 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.

danatt · 12-05-2006, 08:04 AM

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?

Marlin · 12-05-2006, 08:37 AM

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

Quote:

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?

[/b]

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.

DaveinOlyWA · 12-05-2006, 10:29 AM

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

11-25-2006, 04:23 PM	#4
kdmorse Senior Member Join Date: Oct 2005 Location: Germantown, MD Posts: 198 My Car: 2006 Prius Package: #7 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 1	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 __________________ 2006 Black Package #7 Dark Grey Interior Headlight Flasher EV Mod

11-28-2006, 01:53 PM	#7
Charles Suitt Senior Member Join Date: Oct 2004 Location: Dallas TX Posts: 1,622 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	<_< 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. __________________ Charles Suitt Dallas Texas 2007 Touring Edition (Former Prius: 2004)

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11-25-2006, 08:49 AM	#1
moose3720 Join Date: Nov 2006 Posts: 1 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Ok, what's going on here.... 2007 Touring Edition, one week old. Driven (279) miles, fuel gauge was down to two (2) bars, computer says we averaged (39) mpg. However, I could only put (5.1) gallons in the tank before the pump clicked off. Tried two different pumps at two different gas stations, same result. More specifically, the fuel gage showed FULL after only (5.1) gallons! (54) MPG (actual ??) vs (39) mpg (computer average) Also, fuel gage has (10) bars and the car holds (11) gallons, so am I correct to assume that (8) bars should not disappear with the first (5) gallons of gas? Any ideas?

11-25-2006, 09:19 AM	#2
McShemp Senior Member Join Date: Sep 2005 Location: SA, TX Posts: 346 My Car: 2006 Prius Package: #5 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Search the site for tales of the fuel gauge. All of your questions will be answered ... and then some.

11-25-2006, 09:55 AM	#3
efusco Troll Slayer Join Date: Nov 2003 Location: Nixa, MO Posts: 14,710 My Car: 2004 Prius Package: #9 Nominated 1 Time in 1 Post TOTM Awards: 0 Friends: 26	It takes some getting used to and it's definately not something to start sweating over with your first fill up. Try other stations too, I find some pumps routinely cut out early at some stations and my usual station gives me the closest to a 100% fill. The bladder, the guage, and the entire fuel guage system is quite quirky and even buggy, but with time and experience you just get used to what's normal and what's not and how to get maximum distance without running out of gas.

11-28-2006, 04:18 AM	#5
VABeachPrius Join Date: Jul 2006 Location: Virginia Beach, VA Posts: 135 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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.

11-28-2006, 09:33 AM	#6
eagle33199 Platinum Member Join Date: Mar 2006 Location: Minnesota Posts: 3,724 My Car: 2006 Prius Package: #7 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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).

12-05-2006, 08:04 AM	#8
danatt Senior Member Join Date: Nov 2006 Location: Connecticut Posts: 141 My Car: 2007 Prius Package: #6 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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?

12-05-2006, 10:29 AM	#10
DaveinOlyWA Opps !! I Did it Again!! Join Date: Apr 2004 Location: South Puget Sound, WA Posts: 10,010 My Car: 2006 Prius Package: #4 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 6	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