driving my current car (planning to take the lexus plunge shortly) with its cunningly disabled fuel gauge and similarly disabled mileometer/tripmeter (not to mention the drivers window) I have managed on more than 1 occasion to determine that the manufacturers claim of 50l is incorrect, and that the full capacity is in fact almost exactly 55l. (1x 5l jerrycan + 50.5l at the pump 2 miles down the road).
Apparently tho this level of testing is not recommended. Specially on the m25.