Having a fire extinguisher in your car is good, but sometimes it's of no use if car catches an instant and uncontrollable fire. In that scenario, best is to learn - how to exit the burning car with all your "critical belongings" under 10 seconds.
Here are the stats that show the percentage of each type of cause that is responsible for an uncontrollable fire.
49% - Mechanical failure or malfunction
23% - Electrical cause
11% - Leaks or breaks
09% - Unknown causes