How do you know you burned 14 gallons? Did you fill it back up and thats all it took? Or were you guessing based on the fuel gauge?
If you fill the tank, drive to your destination and fill it back up, you can so the math... miles divided by gallons
Anything else and you're just guessing.
This.
That first half of a tank on the gauge is likely not 14 gallons...In the case of my truck, with a 26 gallon tank, if I fill up at 1/2 of a tank, that first half of the tank is about 16 gallons, the second half goes much quicker. If you keep an eye on your gauge I will bet you your first 1/4 tank or so seems to last forever, then after that it goes quicker. This is because as you are filling up, the float on the fuel sender reaches the top of it's limit, yet there is still room to fill up in the tank. If your tank was filled all the way up to the top, this would mean your fuel level sender float would need to be floating on top of that...which would be outside the tank, which is not really possible, so that is part of the reason for that.
As Shawn said, the only accurate way is to fill the truck up, on level ground, until it is full (the nozzle clicks off), drive, then fill up again on level ground until the nozzle clicks off. Divide your mileage by this.
If you were filling it up till it clicked anyway, why didn't you just read the volume on the pump rather than guessing?
On that 24mpg trip, I can about guarantee you that you burned closer to 18+ gallons than 14, which, using 340 mi would drop that 24mpg to a still very respectable for a 6.0 18.9mpg.
