There is another issue that can effect your milage calculation. The fuel they sell today evaporates rapidly. If the truck sits for days at a time or even weeks, you can loose enough to change your numbers.
I'm sure there are some that will not agree with me that it will have that much effect. I tried to check my milage a while back and the numbers were all over, tank to tank. It seemed to make less difference on the type of driving than it did the frequency of the fill-ups.
Starting in 1970, most cars had charcoal canisters. They started in varying degrees to seal up the fuel tanks and the carb. The OBD2 systems starting in 1996, actually self test to make sure no evaporated gas can escape. Charcoal canisters used to return the vapors to the carb when the engine was started. Today the canisters are next to the tank in most of the cars I have seen.
No fuel is lost.
