Why Are You Safe from Lightning in a car?

Due to the Faraday effect electrical current from the lightning strike flows "around" the outside of a conductor instead of flowing though it. The safest place to be is a large well grounded structure with metal that can safely take lightning on a path to the ground. A car fulfills many of these requirements so you can safely survive a strike inside of a vehicle. However, you may still sustain hearing damage do the proximity of the strike. Contrary to the popular myth you are NOT protected by the cars tires. The electricity has already come over a mile through open air so the small amount of rubber contacting the ground will not aid in protection. 

A great example of this is when lightning interacts with aircraft. Commercial aircraft often get hit by lightning. Here is a video of a Delta airlines Jet on the ramp in Atlanta GA. Luckily, because of the metal skin of the aircraft the charge flows around the aircraft body keeping the passengers safe.