Read the links below:
http://www.hanford.gov/fire/safety/e...htm#m-cratings
http://www.fire-extinguisher101.com/agents.html
Suggest a dry chemical B-C rated extinguisher using sodium bicarbonate. This chemical is much less corrosive then other agents and easier to clean up. It is less effective then other dry chemicals thus purchase a larger extinguisher. I have an 8lbs.
Some people like Halon filled extinguishers as they do not leave any residue. Get something at least 2.5 lbs.
Kidde makes a lot of extinguishers.
A larger extinquisher may be in the way on a passenger car compartment, I have it in the trunk. The issue with most car fires is they can get large in a very short period of time, a couple of minutes. But in this short time you can still grab the extenquisher (hopefully large enough) and put it out before it gets larger. Without an extinguisher you can often only watch the car burn.