So what...do you fill the tank, and then drive it until it runs out of gas and see how many miles you've gone?

I take the amount of gas I put in and divide it by the amount of miles I just drove. This isn't incredably accurate for one tank but over many tanks it gets pretty accurate.

Others use the displays

My hydroponics experiment

You ever notice how hard it is to lip read cartoon characters?
"Crazy is what the sane call Delta Flyer"

I'd prefer to go by my car's OBD, but alas... I don't trust it. I calculate my miles per gallon the old-fashioned way... miles divided by gallons.

tbaleno... if i read your comment correctly, you've got it backwards.

My 03 owners manual goes out of the way to warn you not to run the car out of gas, so what you suggested would definitely be a bad idea. I do what the others do - divide miles driven by gallons put in. There is an error margin introduced because the pump may not shut off at the exact same fill levels each time, but given the dozen other variables at play that affect mileage (weather for one), you can only be so accurate anyway.

There was another thread about the display - I think it's safe to say do not trust it. Mine is an average 3.0 gallons off, always on the high side.

*** Retired after 65,000 outstanding miles ***

"divide miles driven by gallons put in"

ummm...so how do you determine how many gallons you've used without driving it to empty?

...by how many gallons you put in when you fill it up.

I track both the calculated data and the display data. I use TripA for each tank and TripB for long term mpg (as long a period as I can before I accidentally reset it - about 4000 miles last time.) The A mpg is usually off by 2-10%, but after a couple tanks, the B display tends to be right on, within a half mpg or so. So... I trust the display for the most part, but go to the calculator for the exact figures. I made a notebook with columns for all the data.

There's been alot of discussions about this over the years across the net.

I think most folks fill it to the first or second click-off, which is what is supposed to be done.
Personally I fill past that point and put all it will take. I do that to fill up less, and since the nozles's click-off point is different than another I believe is a bit more accurate.

I think either one of the above methods will give reasonably accurate results.

Simply running out of gas isn't a good idea as already pointed out, and is still not an accurate measure, if you were to fill the car again at the nearest station.

I belive to get a truly accurate measure you'd have to start with a vehicle void of fuel- gas lines, vapor lines, injectors, pump and the tank itself blown clear of fuel.

Then carefully measure fuel into a calibrated container and pour that specific amount into your verified empty vehcile.
(BTW this is how the first gas pumps functioned a very long time ago)

Obviously that isn't practical.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT

You have to make some assumptions. Fill your tank per your desire (first click, second, overfill, etc.). Drive 5-600 miles, fill again using the same method. The assumption is that the pump will deliver a consistent total amount of fuel, therefore what you put in is what you used over that 5-600 miles. That assumption is where the margin of error comes in. You will never know exactly, because odds are if that pump topped off at 13.1 gallons once, it may top off at 12.9 later. Maybe if you use the same pump at the same station, you'd get close.

*** Retired after 65,000 outstanding miles ***

Let me give this a shot. Hot_Georgia is I think from reading his post doing it like I do (I-Think).

You fill your tank up all the way to the top where if you put in (One-More-Drop) it would run out on the ground. This takes practice and the right pump because you have to try several pumps to find one that will just barely put the fuel out so you can pull the gas handle out of the tank and watch the fuel as it is going into your tank very slowly until it reaches the top of the filler tube and like I said, fill it until one more drop and it would run out on the ground. This takes some practice because you certainly don't want gas running down the side of your car. Thats why I said you have to some times try different pumps to find one that does not have a very strong pumping force that will allow you to put the fuel in very slowly once it reaches the top of the filler tube. This does take some time and patience. Alot-Of-Patience:

Once you have done this drive the car until you put I would think several hundred miles on it at least and then, once again go through the fill up process again.

The amount of gas you just (Refilled-Tank-With) all the way to the top of the filler tube like you did the first time so you know for sure that the tank will not take one more drop of fuel before it drips on the ground.

Now divide the (Gallons Into The Miles Driven) Theres your exact FE (Fuel-Economy) no guess work:

Example: 300/10=30 MPG (Miles-Per-Gallon)
300 Miles Driven, Divided By Gallons You Refill With Equals, AVG. M.P.G. (Average-Miles-Per-Gallon)

HTH

Terry