It's funny that this thread came up, because I am a Fire Extinguisher maintenence Tech for a living. I carry an extinguisher in both of my mustangs at all times, & IMO its very important to have 1 in any modded vehicle...
The type of fire extingusher you want to have is
ABC, because they cover all types of fires. Stay away from BC rated extinguishers because although they do cover grease & electrical fires, a class B or C fire will quickly turn into a class A fire...
Class A Fire= Ordinary combustibles. IE: wood, trash, paper, fabric etc.
Class B Fire= Grease & liquid fires. IE: Gasoline, cooking grease etc.
Class C Fire= Electrical equipment. IE: Fire involving energized electrical equipment such as motors, switches, wiring, where only chemicals that do not conduct electric current are used.
Anyhow, the most common sizes that you will see in vehicles are 2.5lbs but the next common sizes are 5 lbs. usually the type they sell in stores are manufactured by either Kidde, First alert, or strike 1st. Most of these will work in the case of a fire, but these are lower grade brands & most of the time are non refillable after used. Plus, dealing with these everyday, I have seen many that are leakers from the factory & they will lose there pressure.
the BEST brand on the market IMO are manufactured by a company called Amerex. , these are the industry standard, are very durable, refillable, & they are reliable... They cost a bit more, but IMO its worth it for the safety, plus they aren't that much more expensive. Amerex extinguisher's usually run from $32-$38 for a 2.5 lber, & $49-$60 for a 5 lber.
Here is an exapmle of Amerex extinguishers...
Amerex 5 lb ABC...
Amerex 2.5 lb ABC...