How do u guys calculate miles per gallon?

[fuelmiser]

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

[Bill in Houston]

Just try to fill it consistently each time.

[UfoTofU]

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.

[rGS]

Originally Posted by fuelmiser

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...

      Miles Driven between fillups
mpg = -----------------------------
      Gallons of fuel used

So let's use the data you have given to calculate your mpg.

      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.

      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.

[fuelmiser]

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?

[diamondlarry]

Originally Posted by fuelmiser

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.

[minic6]

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??????????

[rGS]

fuelmiser,

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

Originally Posted by fuelmiser

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.

Originally Posted by fuelmiser

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.

Originally Posted by fuelmiser

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?

[fuelmiser]

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.

[landspeed]

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?.

[basjoos]

I'm not that overly concerned with hitting the exact same level with each tank since it all averages out over a couple of tanks.

The way that the fuel cutoff on a gas nozzle works is that there is a small tube inside the gas delivery tube (on the upper side of the nozzle). This small tube passes a piston chamber and then goes to a venturi opening into the gas flow inside the handle. When gas is being delivered, its flow sucks air through the venturi. This air enters the small tube at the nozzle opening. As long as the nozzle is above the fuel level in the tank, this venturi air suck system flows with very little negative pressure, but when the nozzle tip dips into the fuel, it starts trying to suck a liquid up that small tube, whose resistance causes the venturi to start pulling a vacuum, which pulls down the piston that disconnects the finger trigger from the gas valve. You have to fully release the trigger to reconnect the trigger to the gas valve.

[rGS]

Originally Posted by fuelmiser

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...

fuelmiser,

I'm the first owner of my car that I've had since 1997 so to date I've owned my car for 10 years. Over those 10 years, I've come to learn a lot about my car. That also means I've done many fillups over that time. That means I may have become curiously bored while filling up and wondering "how does the nozzle know when to click?" That's when after pulling the gas pump nozzle out of my car, I looked into the nozzle tip and noticed a small movable trigger-like metal tab inside the nozzle tip. Since I felt it would be an unsafe experiment to find out for sure that the metal tab was a trigger to make the nozzle click, the purpose of the metal tab has yet to be proven on an experimental basis. However, I still believe that the metal tab has something to do with a mechanical trigger to make the nozzle click and therefore stop fueling. So, assuming the metal tab acting as a click trigger had to be pushed or triggered off somehow. That's when I thought that there is probably some sort of float based mechanism connected to some sort of stick in the fuel filler neck is responsible for floating up causing the stick to rise and eventually push the metal tab in the pump nozzle tip to communicate mechanically to the nozzle to stop fueling because it's full.

Since you mentioned that you're familiar with what the owner's manual says is the capacity of the fuel tank, then I'm going to assume that you know where the owner's manual is located. Try reading the book cover to cover so that you'll understand your car better than you know it now. Then, obtain a more technical reference of your car where you can find answers to questions like "does my fuel tank and/or fuel filler neck have some sort of float mechanism to communicate mechnically with the fuel nozzle to make it stop fueling?" In summary, learn your car which will lead to knowing your car.

Also, in a safe condition and environment, look inside the pump nozzle tip for the metal tab so you can verify for yourself what I'm talking about.

Originally Posted by fuelmiser

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.

As previously suggested, know your car. Your car's information is more important to know than understanding "all automobiles." Also, as previously mentioned, check out the nozzle tip for the gas pump the next time you fill up.

Unless you're planning on driving somewhere so far where there won't no gas stations until you reach the destination and you're probably going to be coasting into the parking spot running on fumes, I still vote one click as the standard.

Since you mentioned "nit picky" it triggered "the geek within" for my response so if you felt that I came down hard on you like a "ton of bricks" then I'm sorry.

[baddog671]

Just fill it to the first click. That way, the next time that you fill to the first click, whatever the pump says you pumped is exactly the amount of fuel consumed.

And tank capacity doesn't take into consideration the fill neck. So, if your tank is 10ga. and you fill to the rim, it may have been 10.5 gallons..

[diamondlarry]

Originally Posted by baddog671

Just fill it to the first click. That way, the next time that you fill to the first click, whatever the pump says you pumped is exactly the amount of fuel consumed.

What if the pump happens to be pumping faster/slower? It wouldn't be accurate anymore would it? I have noticed a difference in how fast the gas flows when there are multiple people pumping. When they stop, I can discern an increase in flow at the pump I'm using.

[baddog671]

That makes no sense. Whatever the pump says you pumped is the amount of fuel you have consumed.

It doesn't matter how many people are pumping or how slow it comes out of the nozzle, it still displaces the same amount of volume...

[diamondlarry]

Have you ever poured oil through a funnel so fast that it filled the funnel before it had a chance to drain into the container you were filling? The same thing can happen at a gas pump. When the fuel level rises to the point in the filler pipe that it triggers the shut-off mechanism the pump clicks off. If the pump is pumping a bit faster, the pipe will fill faster but there won't be as much gas in the tank as the last time.

[zpiloto]

Wow quite a bit of heavy thinking going on. The more fuel you put in the tank the more accurate it will be. Just log all fill ups and try and use the same pump and the same method to fill every time whether it's first click or top off. The pump and fill inaccuracies will even out in the long run if your logging all fill ups.

[Bill in Houston]

Consistency is the key. Do whatever your experience tells you gives you the most consistent fill.

[ma4t]

Here's what I do. I hit the trip meter every time I fill up. So I start with zero miles each tank. If I go 300 miles before it's time to fill up, I write that on my receipt and mark which car I was driving (since we have 2).

When I get home, I input the number of miles travelled and the number of gallons bought in my fuel log. (I love the fuel log here because it allows me to get an accurate MPG calculation and it allows me to chart my progress.) Before that, I just did this:

(Miles travelled) divided by (gallons bought) = mpg

I don't worry about how many gallons my tank can hold.

In addition, I keep track of little things that are going on during the time I used the gas on the receipt. How was the weather, driving conditions? Did I change the oil? Did I try something new? Reduce weight, etc? That way, if I removed something really heavy and my FE shot up, I know what it was with reasonable certainty.

Not everybody is as compulsive as me about keeping records.

Have fun!

m

[fuelmiser]

i appreciate everyone's reply to this thread. thank you! it gives me an idea of everyone's thought process. i didnt know the fuel tank area was so complex. but from what im gathering..i conclude that consistency is the most important variable when filling up.

one thing noone mentioned is...after filling up the tank...go to each four corners of the car and give each corner a couple of bounces to get rid of "Air Pockets"...is this myth true? things that make u go hmmm...

[diamondlarry]

Originally Posted by fuelmiser

i appreciate everyone's reply to this thread. thank you! it gives me an idea of everyone's thought process. i didnt know the fuel tank area was so complex. but from what im gathering..i conclude that consistency is the most important variable when filling up.

one thing noone mentioned is...after filling up the tank...go to each four corners of the car and give each corner a couple of bounces to get rid of "Air Pockets"...is this myth true? things that make u go hmmm...

I wouldn't say that you have to go to all 4 corners but I do shake my car by grabbing the fenderwell and quickly shaking it up and down. I keep doing this until the level in the filler doesn't change for at least 10 seconds after the shaking. I just like to be consistent.

HEK, interesting thought on the ramp thing.

[rearviewmirror]

https://www.fueleconomy.gov/mpg/MPG.do

[Bill in Houston]

You could easily make a little ramp 4 inches tall from two pieces of 2x4, and just lift the corner or the car where the fill port is. Not worth the hassle to me, though.

[Bennet Pullen]

rGS, wouldn't the temperature related expansion/contraction properties have just as much a factor on accuracy using the first click method as the top off method? Overflow concerns aside the gas will still expand and contract the same no matter which way you fill your tank.

Responding to diamondlarry, one of the local news stations in my area did a piece on gas pump accuracy last summer when gas prices spiked. They found that the slower you pump the gas, the more accurate the gauge was. Keep in mind they were filling up graduated cylinders with gas and weren't even using the automatic cutoff. So I guess I would say yes speed makes a change, I personally always use the first (slowest) hold position on the nozzle.

I also fill up using the "second click" method, where you fill to the first click, wait a bit for bubbles, waves, etc to clear and fill slowly to the click again. I think this method will be more accurate (especially between pumps) than the first click because it takes out some of the variables of frothing etc.

[diamondlarry]

Originally Posted by Bennet Pullen

rGS, wouldn't the temperature related expansion/contraction properties have just as much a factor on accuracy using the first click method as the top off method? Overflow concerns aside the gas will still expand and contract the same no matter which way you fill your tank.

Responding to diamondlarry, one of the local news stations in my area did a piece on gas pump accuracy last summer when gas prices spiked. They found that the slower you pump the gas, the more accurate the gauge was. Keep in mind they were filling up graduated cylinders with gas and weren't even using the automatic cutoff. So I guess I would say yes speed makes a change, I personally always use the first (slowest) hold position on the nozzle.

I also fill up using the "second click" method, where you fill to the first click, wait a bit for bubbles, waves, etc to clear and fill slowly to the click again. I think this method will be more accurate (especially between pumps) than the first click because it takes out some of the variables of frothing etc.

I think that I agree that this would be somewhat more accurate/consistent than the 1st-click method. Foaming/turbulence would be much less of a factor using this method. I usually don't even go all the way to the first notch when I'm pumping my gas unless I'm in a hurry which rarely is the case.

[rGS]

Originally Posted by Bennet Pullen

rGS, wouldn't the temperature related expansion/contraction properties have just as much a factor on accuracy using the first click method as the top off method? Overflow concerns aside the gas will still expand and contract the same no matter which way you fill your tank.

Bennet Pullen,

It's a matter of the laws of physics limit called "maximum volume" in other words 100%. So let's say you fill up the tank to 100%. Then the gas expands. Since the tank can't physically expand proportionally as fast as the gasoline so the gas can remain in the tank and therefore no wasted gas, the reality of the situation is that the gasoline WILL overflow. That's because the material property of gasoline expands and also contracts at a faster rate than the expansion and contraction rate of the metal used to make the gas tank. That overflowed gas will literally be a WASTE which is effectively the same thing as driving non-fuel efficiently which we're trying to avoid. So, once the fuel is wasted it's wasted and that means wasted money on gas that you didn't use.

However, by using the first click method, you fill your gas tank up to the "full point" that was calculated by the engineers so that even with gas expansion due to hot temperature, the gasoline shouldn't expand more than the capacity of your gas tank. What that means is that more of your gas remains in your gas tank and therefore is NOT wasted. Therefore since the gas is NOT wasted your money you paid for the gas isn't wasted either.

[Bill in Houston]

This thread totally went down the rabbit hole, huh?

[zpiloto]

Originally Posted by Bill in Houston

This thread totally went down the rabbit hole, huh?

No kidding. Do we take the red pill or the blue?

[brucepick]

Yes it went down the rabbit hole.

Anyway, I'm with rGS.
I like a nice simple fill to the first click. I fill kinda slow to avoid lots of chaos going on inside the tank. Also to avoid splash that will be pulled back out via the vapor recovery system (just a theory of mine).

I think this is probably more consistent than using two clicks or till you see gas at the top of the fill tube. Not much is worse than spilling the fuel. Duh! Any inaccuracies should even out pretty easily.

[96hb]

Are these few tenths variations really going to make that big a difference? I don't think so. Whatever way you choose to fill up, just do it consistently and you will be fine.