Ferry companies get very ansty over jerrycans, I normally leave the top open. Also would you really like twenty litres of fuel sitting right in the impact zone of a crash? Plus unleaded fuel decays very quickly. Not to mention the story of the lady today decanting fuel. Nah, when mounted on vehicle, show only! As for petrol fuel storage, I've always belived no more than 20 litres may be stored in a purpose built metal container, and 5 litre in a purpose built plastic container. Get Disiel no problems there.