How do u guys calculate miles per gallon?

[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.

[GasSavers_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

[GasSavers_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...