Ive cn those b4, but i think they fill them with water/antifreeze solution to use as weight of havin a car full of people durrin thier testin. I c those test cars all over here in fairbanks alaska. Have a friend that works for the company that does the testin. Ill ask him about those and c what he says they put in them. But i know i would put one in my plane. Hahhaaaaa