Just a thought......Gasoline evaporates very quickly (much faster then water). When you fill up on a completely empty tank some of the gas is going to evaporate before your tank fills up. This could explain the difference you see betw. gas used and gas filled at the pump. Also remember that some gas stations dont calibrate their gas pumps correctly. This is illegal but many gas stations dont care because the rules are rarely enforced and they make more money this way. i.e. if the pump says your car got 13 gallons but you really only received 12.5. The gas station just made the $ on the spread.