Just a dumb question maybe, but is there more than one definition of a gallon

?
here:
http://www.wolframalpha.com/input/?i=11+gallons+in+litersthey say 11 gallons are roughly 42 liters. Which also roughly matches the fuel capacity of the 2009 two-seater. My max fill-up was 41 liters, and the needle was not completely down to zero yet.
However, the Company in their advertising material of 2008 says that the fuel capacity of the Plus 4 is
"55/12 (ltrs/galls)".
While I have the paperwork

here: the Plus 4 drive ratio in fifth gear is 0.76 and the axle ratio is 3.73. That yields 25.1 mph per 1000 prm (if you believe in their numbers).
Andy