Overfilling theory (input needed)
 

Mind you, this is only a theory that I would like the general opinion from Fuelly users, who tend to be more mpg driven and conscious than the general population.

As the adult son of a 40+ years mechanic, who also tinkers in his own cars, I completely understand why overfilling the gas tank is a bad thing. But it strikes me that it would only be bad on an everyday scenario, and specially if you overfill the tank and then park your vehicle for several hot summer days (please don't do that).

My theory is: If you are about to take a looooong road trip, lets call it El Paso to San Antonio, If you overfill your fuel tank as the last thing you do before leaving town, wouldn't the "extra" fuel you are carrying be used-up before it has a chance to evaporate, and since you are driving at highway speed, the catalitic converter would be able to burn off any additional fumes as it is intended to anyway? Also on said road trip, wouldn't you be able to get some additional range?

If you do it now and then, I'm sure there's no harm. I did it every fill for over a year and I'm lucky that it didn't damage my canister. The only reason I did it was to provide extra accuracy on my fillups. I decided the risk wasn't worth the extra accuracy or the extra ~.8 gallons of extra fuel.

I always brim mine until you can physically see fuel sitting right by the cap itself. It says in the manual to stop after the third click, but I'm so obsessed with getting the most miles out of my tank, Im not fussed about any risks. I like being able to do 320 miles before the fuel gauge starts to drop.

You might find people that do this are the same people that drive forever once the fuel low warning light appears too, yep I'm guilty of that aswell, I generally leave it a week or so when the light pops on, some people believe that running fuel that low is bad for your car too. Like you say, there are lots of theories out there, some prove people right, some prove people wrong it all depends if you want to risk it or not. I personally don't feel there's any harm in practicing either of these methods.

most vehicles have a"charcoal cannister" that catches the vapor (fuel) & can be "saturated" do not know if that is thecase of your vehicle, I usually stop when the cutoff stops the flow of fuel - good practice?

I'd say it's good practice to stop at the first click. It's why your filler cap and the gas pump will say do not top off. As I said, I miss the extra range and the (probably, maybe) more accurate fill data from stuffing the tank, but it's not worth the possibility of having to prematurely replace the canister. In mine, it's a $300 part for a car worth maybe $800.

Not sure if diesels even have a canister? Diesel doesn't evaporate like gasoline does.

It really doesn't matter very much whether you top off the tank or not, as long as you are reasonably consistent. Fill the tank till the nozzle shuts off and call it good; fill it to the next convenient unit of currency (dollar, in the USA); or waste time trying to get the very last drop in the tank. It will all average out within a few tanks anyway. Your day-to-day variations in driving, the weather changes over even a few days - or worse, over a month - will have far more effect. Seasonal variations in fuel mix, coupled with the uncertainty of what you get (is the service station tank all winter blend, all summer blend, or some unknown mixture? how much ethanol is in it?) make a variation of a fraction of a gallon unimportant.

Just hate to fill at a gas station. Fill the tank up!!! That's why diesel VW's were so tempting. 900 highway miles before you need filling. Was interested in an ECO Cruze, which had a false specification of a 12.6 gallon tank, but really had a 15.6 gallon tank. Carefully driven highway range would be 700+miles. Tho my car was bad-mouthed for bad mpg, I've been able to average 39mpg & keep the range above 400 miles, which is nice. Fill the tank up!!!

Elantras are fantastic hypermilers. Doesn't take much more effort to keep its range over 500mi. Great cars!

I see the discussion seems to have shifted from getting the most accurate mpg figure for a given fill to getting the longest range on a fill. For range, the semi I used to drive averaged about 6 mpg but had a range exceeding 1100 miles at mostly highway speeds. I never ran it dry and usually filled it about once per day at somewhere around 500 miles. Its secret to long range was simple - two saddle tanks holding over 100 gallons each.

The efficiency of large trucks has significantly improved since then, it's not unusual to get 12 to 17 MPG in a large semi these days, and the long life oils used can do 100,000 km between changes, better for the environment all round. There's also less need for such big tanks, around here, fuel is so dear, trucks had to have armour plated tanks to stop people drilling them and siphoning it off. A tank that size fully loaded with fuel could be worth up to $2000 her, easy target.