Helium is not flammable, but filling your car tires with it would have the following effects depending on the conditions. a little technical but Helium is an enert permiable gas and would leak from the inner lining of the tire at about 10psi per hour or so. On the reverse if the car is driven at high speeds the gas molecules speed up and create an unstable internal pressure and at some point if you go over even a tiny piece of rock the wrong way Kaboooom!! Its just like a helium balloon, leave it in the heat and it will explode, but if you just leave it around in normal tempretures no matter what eventually it goes flat.