First.... I HIGHLY RECOMMEND that a fire extinguisher be fastened and secured to something with a quick release extinguisher bracket or velcro! You do NOT want a 2, 5, or 10 lb. extinguisher flying around inside a vehicle!!
A good high quality 2.5 lbs ABC should be adequate for an auto fire, or campfire fire, etc. A 5 lb. one would be my choice.
If you are overly concerned about possible damage caused by ammonium phosphate, you can always heat up your credit card with one filled with halon. 
If you have ever been in a fire or had to discharge a fire extinguisher you know how short of time they actually work.... typically 5 to 15 seconds. A fire can be very "large and inconvenient"!
Gnarls. 
Yeah I'm aware. My "setup" sucks but at least I've got one and so do the people I wheel with. 5 Lbs is what I have now.
I had an electrical meltdown from a shorted coil wire once. Happened about an hour and a half into the trail from the nearest service road. Fire extinguisher wasn't needed (Thank God
). Had to get hauled out by a Yj, probably the proudest day of that jeeps life
. Definitely inconvenient but all things considered it was still a good day.
Anyway, The Element E50. 50 sec. discharge, small package, clean/non corrosive, non toxic, not effected by weather, no maintenance, no expiration.
I probably should order a couple of these.
