Top Gear tested this a couple years ago:
As you can see, the car body acts as a lighting rod, directing the electricity around the outside of the vehicle and away from you. It's very unlikely that the electricity will pass through you unless you are touching something metal connected to the car's exterior. The current goes through the air between the car body and the ground, not the tires. While you may be unharmed, the electrical system may be damaged, and there may be scorch marks in the area of the strike.
I've personally experienced lightning strikes twice, both while driving and outside a vehicle. In the first case, I was in an old truck with no engine electronics. I was temporarily blinded by the flash, and the engine shut off. I was able to pull over and restart the vehicle, with no discernible damage. The second time lighting struck a car parked outside my house. It destroyed the control module for the radiator fan.
If you do find yourself driving in an electrical storm, pull over and stay inside your car. Place your hands in your lap and wait: as long as you aren't touching any metal part connected to the body, you should be safe.
Posted 2728 day ago