Just had a look at dielectric strength of air. Consensus of the pages that I've looked at so far is 3,000,000 V/M. If we go with the voltage number presented above then your tires will need to be about 84 meters in diameter before they would have a chance of raising the vehicle far enough off the ground to insulate it. This assumes no steel plys in those tires.
Otherwise a lightning strike will have enough voltage to ionize the air between the metal of the vehicle and the ground and jump the gap.