Is a car a safe place to be in a thunderstorm? Most of the time, it is. At least, it is a pretty safe place to be.
But the reason for this has nothing to do with the tires. A lightning bolt connects opposite charge between cloud and ground through thousands of feet of air. A half inch of synthetic rubber (belted with steel as well) is not going to stop it.
Rather, if a car becomes the path for lightning, the current will tend to flow around the outside of the vehicle, using whatever conducting material is available, leaving the inside mostly unaffected.
Often, lightning strikes a car with no effect other than a bang. Other times, the entire electrical system of the car is ruined. Although it is rare, people inside a car can be electrocuted. But given the choice of riding out an electric storm inside a car or out in the open, a car is the much safer option.