Fuel Tank Capacity...
I saw a 20/20 program about gas stations that were running a short fill scam in San Diego. The way it worked was the pump dials would run faster than the actual pump until it approched 5, 10 & 15 gallons, then they would run slower until the actual amount caught up. This way if you bought gas in increments of 5 galons it would be correct. But if you bought any where in between it would be off by as much as 15%. Clever way to get by the weights and measures people but still cheat. I believe that this was in the early 90s. In short don't fill one five gallon can but fill five 1 gallon cans to test the system.
Most likely you are filling the tank and the filler neck to the max without leaving any expansion room. The practical fill volume is what is listed in the owners manual. This would be the amount that would leave a small amount of space for expansion and the filler neck would not be included.
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