A car is safe in a thunder storm because of the metal shell you are sitting in. The metal directs the lighting around you to the ground, it's called a Faraday shield. A friend of mine was hit by lighting while driving down the interstate at 65, it melted the antenna and blew out a tire. Everyone in the car was uninjured.
Tires do not insulate you from the ground it would just jump the gap after it fried you if you where hit on your bike.