An Imperial gallon is 4.54 litres. A U.S. gallon is 3.79 litres. Multiply miles per Imperial gallon by 3.79, and divide by 4.54 to get miles per U.S. gallon.
I think.
The last time I checked mine, I got about 17.5-18.5 (U.S. gallons). My truck had 34,000 miles on it at the time, Juice...