OK, something I know a little bit about.
First, no fire extinguisher does everything well, you have to figure out what the goal is.
Hint: electrical conduction fires "C" are usually not serious if you can remove the power. The exception is if the electrical fire causes another type of fire. Fires that leave an ash "A" are pretty easy to put out and if you are working on a car fire type "A" materials are carpets, seats, and human beings (we used to call them screaming alfa's). The biggies are flammable liquids "B" and flammable metals "D". The liquids are oil and gas, of course. The metal is magnesium. We have engine parts made of magnesium and there is no way to put out a metal fire because they make their own oxygen and they are very hot.
Second, let's concentrate on liquid fires, the ones that destroy our cars. Aqueous Film Forming Foam (AFFF) is the best but very hard to carry around in the car (that was mentioned before). Also, it has a shelf life and you need enough to prevent a re-flash. I have seen a guy step in a puddle of it and the fire erupted when he lifted his boot. Finally, it has no use on electrical fires and almost useless on type "A" fires. The gases: CO2, Halon, and water fog spray work pretty well but you have to get the fire completely out at close range and hope the surfaces are cool otherwise Poof, a re-flash.
Third, what to use? Powder, but not just any powder. Baking soda works in the kitchen but sodium bicarbonate is not very good at removing the oxygen (smothering) it is slightly better than salt. So we want something that smothers the fire, does not conduct electricity, and not as corrosive as some other agents. General purpose dry chemical extinguishers use monoammonium phosphate, nasty stuff. So, use what the Navy uses on their ships, Purple K Powder (PKP) or potassium bicarbonate.
From Wikipedia:
"Purple-K is a dry chemical fire suppression agent used in some dry powder fire extinguishers. It is the most effective dry chemical in fighting class B (flammable liquid) fires, and can be used against some energized electrical equipment fires (USA class C fires). It has about 4–5 times more effective against class B fires than carbon dioxide, and more than twice that of sodium bicarbonate. Some fire extinguishers are capable of operation in temperatures down to −54 °C or up to +49 °C. Dry Chemical Powder works by directly inhibiting the chemical chain reaction which forms one of the four sides of the fire tetrahedron (Heat + Oxygen + Fuel + Chemical Chain Reaction = Fire). To a much smaller degree it also has a smothering effect—excluding oxygen from the fire."
I have a 10lb bottle of PKP in my car (it fits on a Brey-Krause mount). If you ever have an engine fire, you will be glad if I pull up behind you because I have a 20 foot range and 24 seconds of discharge time. You can get PKP extinguishers at places like this:
Amerex 10 lb Purple K Fire Extinguisher
and
Ansul Sentry 10lb. Purple K Fire Extinguisher - Monroe Extinguisher
More expensive that a 2.5 lb can of baking soda, but inexpensive insurance to me.
Mark