I have learned to carry two fire extinguishers in my car after a recent incident in the underpass at the LAX airport. There was a van stopped in the tunnel with the hood up and smoke coming from it. No sirens to be heard, so I pulled up and took my 5 pound dry chemical extinguisher from the trunk of my car and made quick work of the fire.
Upon going to get it recharged I was told it was out of the hydro test date and it could not be refilled without being hydro'd. So, I shelled out the money for a new replacement. I think the hydro is good for 12 years.
The act of kindness cost me about $50, but I am happy I did it.
I also bought a $10, 2 1/2 pound disposable extinguisher that I will use first if called upon again. Cheaper than using the 5 lb unit, as long as it will do the job.

Also, I suggest that you purchase one with a hose, it offers much more versatility than a straight nozzle type unit. Whatever you buy, secure it in your vehicle, as it can become a nasty rocket in an accident.