Simple. Water is heavy, as in, 1 kilo per litre. Air, and sometimes some kind of gas (can't remember if its outlawed now….) is light. Race cars, as a rule of thumb, are lightweight mean machines that somewhat look like their common, everyday commuter, say, a Honda Civic. read more
Water filled tires add lots of weight they don’t want. Water in the tires would affect some other things as well. Two things come to mind. One is the added centrifugal force on the inside of the tire. The other is the the ability of the tire to heat up. Race cars need hot tires to a point and that point is where the rubber becomes sticky. read more
Many race car teams use nitrogen instead of air in their tires because nitrogen has a much more consistent rate of expansion and contraction compared to the usual air. Often, a half pound of pressure will radically affect traction and handling. With track and tire temperatures varying over the duration of a race, the consistency of nitrogen is needed. read more