I keep halon (or whatever the replacement is to halon) extinguishers in each car within reach of driver belted in the seat. I've seen too many incidents working at race tracks over the years to do otherwise. I like the halon as it avoids the caustic powder mess if you are only dealing with a small fire.

Whatever you do, if you carry an extinguisher in a car, use a mounting bracket. One designed for mobile applications, not a cheap plastic wall hanger. A fire extinguisher becomes a lethal missile in a collision.

Craig.