Hey all, I'm actually a firefighter from brissy and I would only recommend using water as a last resort on a vehicle fire.
I've put out numerous car fires and a small 1kg extinguisher is useful only in the initial stages of a vehicle fire, such as small fuel and electrical fires etc.
If the vehicle fire is in the engine bay chances are that is a fuel or oil fire to start with which can be put out with a dry chemical extinguisher but if it escalates and the engine block begins to burn it is usually too late to save.
Magnesium in engine blocks and steering wheels is highly volatile and reacts violently with water causing it to vaporize and explode molten metal everywhere!
We do extinguish car fires with water and a foam mix and most of the time we do not save the entire car as by the time we get there from the call it is far too late as car fires do burn fiercely. I always have a 1kg powder extinguisher on board and have a fire blanket as well, you can never be too careful as it is cheap insurance when a firetruck is usually at least 8mins away.
These small extinguishers don't last long as Jason said 10secs, so they are capable of extinguishing a small area and can be used on Wood, paper & cloth, flammable liquid fires and electrical based fires.
I hope this helps?