A.) The "2.5 Gallon Tank" is not properly calibrated. I've never found a container of gas that contained the amount at the "full" line that it said it contained.

B.) Beyond that, FULL for a 2.5 Gallon tank is only filling approximately 2/3rds of the total volume of the cannister, this is to ensure there's enough room for Gasoline Evaporate to not cause problems.

C.) Pumps are governmentally regulated and calibrated, they can be off, or have a bad regulator, but typically this would make a 5% difference at most. Trust me, if you were right about this pump, the guy behind the counter would know what the hell you're talking about because the first person to drive up in a Suburban and have to put $200 of gas in it to get 3/4's of a tank would be flipping his **** at how his tank suddenly held an extra 15 gallons. Since Gas is taxed as an excise, the government has to measure precisely how many gallons of gas are sold. There aren't that many inspectors, so **** happens, but it's pretty nasty penalties if you get caught with that, considering it's fraud.

There's three perfectly intelligent points you should consider about this whole fiasco before you continue to simply assume the operators were ripping you off.