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Old 06-29-2007, 08:00 PM   #1
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How do u guys calculate miles per gallon?

I have a 15.3 gallon tank according to my owner's manual.

On my last fillup I accumulated 322 miles (counted by odometer) and filled up 13.0 gallons exactly (gas pump receipt). This gives me a 24.769 mpg. Now when I filled up...I did continue to add more until I saw fuel on the top of the opening. I did not spill by the way. Is this how u calculate your mpg?

Now my other question is...does the 15.3 gallon specification include the neck area of my car? The reason I ask this is because i continued to add fuel past the initial automatic turn off by the gas pump. Is there a better way to calculate? Should I use the scangauge?

I know im a bit anal when I ask these questions but hey..where else am i gonna ask these questions? Just want to be as accurate as possible
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Old 06-29-2007, 08:26 PM   #2
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Just try to fill it consistently each time.
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Old 06-29-2007, 08:29 PM   #3
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I just let the pump fill until it reaches the auto shut-off.

Divide the number of miles traveled by gallons purchased and figure it that way.

I do always try to go to the same pump at the same gas station if possible.
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Old 06-29-2007, 08:59 PM   #4
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Quote:
Originally Posted by fuelmiser View Post
I have a 15.3 gallon tank according to my owner's manual.

On my last fillup I accumulated 322 miles (counted by odometer) and filled up 13.0 gallons exactly (gas pump receipt). This gives me a 24.769 mpg. Now when I filled up...I did continue to add more until I saw fuel on the top of the opening. I did not spill by the way. Is this how u calculate your mpg?

Now my other question is...does the 15.3 gallon specification include the neck area of my car? The reason I ask this is because i continued to add fuel past the initial automatic turn off by the gas pump. Is there a better way to calculate? Should I use the scangauge?

I know im a bit anal when I ask these questions but hey..where else am i gonna ask these questions? Just want to be as accurate as possible
fuelmiser,

Firstly, I'd recommend that when you fill up that you stop at the first click and do that consistently from now on. Here's why.

Fuel economy is measured in units of mpg. The mathematical definition of "mpg" is "miles per gallon" or as follows...
Code:
      Miles Driven between fillups
mpg = -----------------------------
      Gallons of fuel used
So let's use the data you have given to calculate your mpg.

Code:
      Miles Driven between fillups   322 miles
mpg = ---------------------------- = ------------- = 24.769 mpg
      Gallons of fuel used           13.0 gallons
Now, here's the problem with "topping off" and how it messes up your "gallons of fuel used" figure. Let's say your gas tank is full at 15.3 gallons. Now, let's say you drive your car around. Then, when you fillup to the first click, your gas tank is back to 15.3 gallons. So that means "Gallons of fuel used" = "The amount of gas you filled up with to first click." But, if you top off, "Gallons of fuel used" != "The amount of gas you filled up with to first click" + "top off amount." Mathematically, this would cause you to have a worse fuel economy that you actually got because the denominator is bigger than it needs to be. Let's say you topped off 0.2 gallons. So that means the actual gallons used was 12.8 gallons instead of the 13.0 gallons you topped off with. Let's see the effect if you only went to the first click.

Code:
      Miles Driven between fillups   322 miles
mpg = ---------------------------- = ------------ = 25.15625 mpg 
      Gallons of fuel used           12.8 gallons
Soo, as you can see, if you DIDN'T top off, your fuel economy would have been 25 mpg instead of 24 mpg.

So in order to "flush out" the "top off error" factor, you're going to need to drive your car around until you've burned off more than the "additional top off" amount of fuel that you filled up with, then fill up your tank again to the first click. Now you would be able to calculate a more accurate fuel economy value as well as being able to compare them with each other.

Without knowing the specifics of your car, I'd imagine the 15.3 gallon capacity may be for your fuel tank itself. So the capacity of the fuel filler neck may not be included in the 15.3 gallon figure.

Since I don't have a ScanGuage yet, I'm not sure how it would help to calculate your fuel economy between fillups. I do know that it helps to know your instantaneous fuel economy to help you modify your driving habits.

I hope this helps.
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Old 06-29-2007, 09:20 PM   #5
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Previous post...."Mathematically, this would cause you to have a worse fuel economy that you actually got because the denominator is bigger than it needs to be."

Yes I am aware of the math for calculating mpg but what if I fill up again by topping off since my last calculation was a "topped off" calculation.

Anyways, but isnt it better to calculate mpg when u top off instead of the first click.

My reason behind this is because when u top off....you can see the fuel in the opening. Conclusion: You can see the fuel is "exactly" at that level.

First click...what is first click anyways? what does it mean? Can u see fuel at the exact same level every time u fill up to the first click? I can understand that first click calculations will be more consistent if you pump at the same station and same pump but who does that?

Topoff is better because you have a visual on the exact fuel level when you fill up. The visual of fuel is your confirmation for a consistent calculation.

I know this is a very picky discussion but ehh...why not?
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Old 06-29-2007, 09:47 PM   #6
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Quote:
Originally Posted by fuelmiser View Post
Previous post...."Mathematically, this would cause you to have a worse fuel economy that you actually got because the denominator is bigger than it needs to be."

Yes I am aware of the math for calculating mpg but what if I fill up again by topping off since my last calculation was a "topped off" calculation.

Anyways, but isnt it better to calculate mpg when u top off instead of the first click.

My reason behind this is because when u top off....you can see the fuel in the opening. Conclusion: You can see the fuel is "exactly" at that level.

First click...what is first click anyways? what does it mean? Can u see fuel at the exact same level every time u fill up to the first click? I can understand that first click calculations will be more consistent if you pump at the same station and same pump but who does that?

Topoff is better because you have a visual on the exact fuel level when you fill up. The visual of fuel is your confirmation for a consistent calculation.

I know this is a very picky discussion but ehh...why not?
I agree with you, fuelmiser. I think there are too many variables when doing the "first click" method. If you top off, full is always full. As for the argument that it can cause problems with the evap system, I have topped off with every car I ever owned and never had any problems with the evap. The cars include a '67 American, '72 AMC Hornet, '74 Hornet, '76 Pacer, '83 Reliant, '87 Horizon, '95 Neon, 96 Neon, '98 Durango, 2 '97 Saturn SL's, '99 Saturn SL2, and an '07 Prius. I should also note that I live about 5 miles from the nearest gas station but, in the past, have lived as close as 1.5 miles from a station.
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Old 06-29-2007, 10:03 PM   #7
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I''m very lucky in the way the metro is filled. It is straight up and down so I do the visual and top off to the exact same spot each time. Works well for me. Don't worry about over filling as to damaging your charcoal canister. If you drive for awhile after topping off. I usually drive between 10 to 20 miles or park in the garage not allowing the fuel to expand to much. Shutting of after the first shut off I'd be leary of as to the speed of the fill the expansion rate of the fuel on a particular day??????????
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Old 06-29-2007, 10:14 PM   #8
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fuelmiser,

If you wanna be picky, let's get picky.

Quote:
Originally Posted by fuelmiser View Post
Previous post...."Mathematically, this would cause you to have a worse fuel economy that you actually got because the denominator is bigger than it needs to be."

Yes I am aware of the math for calculating mpg but what if I fill up again by topping off since my last calculation was a "topped off" calculation.

Anyways, but isnt it better to calculate mpg when u top off instead of the first click.

My reason behind this is because when u top off....you can see the fuel in the opening. Conclusion: You can see the fuel is "exactly" at that level.
The problem with the visual method is fuel density in relation to atmospheric temperatures. A gallon will have more molecules of fuel at relatively lower temperatures than compared to higher temperatures. So if you get a gallon of fuel when it's hot outside then it'll "shrink" to less than a gallon later on at night when the temperature drops. Conversely, if you fill up when the temperature is relatively cooler then it gets relatively hotter, the fuel will expand. So let's say at the crack of dawn which is atmospherically the coldest time of day you fill up your fuel tank using the "visual method." Then, the air warms up until high noon. Well, that temperature rise would cause your gas to expand in volume. Assuming "visual level" is at the filler neck opening, that gas expansion would result in your gas to spill out and become wasted and unburned/used gas. But you say, you've got a gas cap so the gas couldn't spill out and get wasted. Well, with some of the fluid systems that I know, there's going to be some sort of overflow vent system to prevent dangerous levels of pressure to build up. In the fuel system, if the fuel tank isn't overfilled, then the overflow vent system may release the air pressure above the fuel. But if you overfill your gas tank so there may not be any air above the fuel, guess what's gonna come out of the overflow vent? Yep, wasted gas. So instead of thinking of the fuel filler neck as an opportunity to top off, let the fuel filler neck be the fuel expansion path.

Quote:
Originally Posted by fuelmiser View Post
First click...what is first click anyways? what does it mean? Can u see fuel at the exact same level every time u fill up to the first click? I can understand that first click calculations will be more consistent if you pump at the same station and same pump but who does that?
Look carefully at the gas nozzle end next time you're at the gas station. For the gas nozzles I've seen, there's a trigger located inside the nozzle tip. My reasonable guess is that trigger is what makes the nozzle go "click." I further believe that there's some sort of float mechanism in the fuel filler neck that reaches out and pushes that trigger when the fuel tank is full. So to answer your question, I believe that click means "fuel tank is full." Since my fuel filler neck has a vapor recovery mechanism to include a self sealing spring loaded fuel nozzle door, no I cannot see the fuel level based on visual inspection of the fuel filler neck opening. Since I use the same fuel filler neck, I'm assuming the "fuel tank is full" float mechanism should be operating consistently therefore consider that my fuel tank is full when the nozzle clicks the first time. I will admit that I do not use the exact same pump at the exact same gas station all the time, but I understand that each and every gas pump in my state must be certified by the "weights & measures" department of government which is good enough for me for now that the pump actually gave me the quantity of fuel as indicated by the "gallons" counter on the gas pump. I also assume the trigger located inside the nozzle is also calibrated to indicate a proper "tank is full" indication click.

Quote:
Originally Posted by fuelmiser View Post
Topoff is better because you have a visual on the exact fuel level when you fill up. The visual of fuel is your confirmation for a consistent calculation.

I know this is a very picky discussion but ehh...why not?
I have clearly showed mathematically the effect on your fuel economy calculation may not be accurate if you use a top off method. So due to the built-in error in your data, that error continues to exist in your fuel economy calculated figure. The error in fuel economy calculation will make comparing your fuel economy with anyone else meaningless because then you'd be doing the classic "apples & oranges" comparison.

So I still don't believe that "visual confirmation" is a way to calculate your fuel economy more accurately. As the math shows, it is better to have a lower gallons used/filled quantity than the relatively higher top off gallons filled that doesn't equal the actual gas used.

Do you now still believe top off is the better method?
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Old 06-30-2007, 03:12 AM   #9
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rGS,

float mechanism? trigger in the gas nozzle? vapor recovery mechanism? overflow vent? whoa! i dont even know about these things! im a newbie here so I am learning! but these other variables you mentioned are interesting...

are these all standard items for all automobiles and gas pumps? I guess my point is...I want a mainstream calculation of fuel economy that the average person can understand that is applicable to everyone. There are too many varables if you are going to consider all of those things above. I could be wrong since I am not experienced with this.
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Old 06-30-2007, 03:16 AM   #10
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Fuelmiser - here is what I do:

(1) Try to park on level ground (if possible, use the same pump at the same gas station).

(2) Guess how much fuel you will need from the position of the gas tank needle

(3) Fill the tank as fast as usual, but then slow down as you approach your estimate. Go quite slowly in filling, until it 'clicks'.

(4) Wait for 10 seconds (to reduce 'waves' and foam in the tank.

(5) With the gas pump still pushed all the way into the filler hole, very slowly fill the car, until it clicks again.

If you do this, it will be pretty accurate - as you will reduce the random effect of waves / foam in the tank.

This works on UK petrol pumps - I presume it is the same on US gas pumps?.
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