A few points from a fire nut who has put out a few.
Whatever extinguisher you get, make sure your logistics budget
includes money for two or three so you can practice with them. A good test for me is a large old tire or worse yet a cardboard box with 5 gallons of diesel;/gas mix dumped on it. You can put something like that out with a handheld, but you better be good. If you have not actually put out a few fires with your chosen weapon I have no idea why you have a fire extinguisher, kinda like carrying a gun you never fired.
*Compliance with local smog laws, fire codes, nosy neighbors, and not blowing yourself up is your problem*
-Dry powder is an excellent improvised defense spray.
-Dry powder is also good for most auto fires with several caveats:
1. It offers no reflash protection as in the fuel is not separated from air or ignition source. The last car fire I almost put out I could hear the electric fuel pump running, feeding a severed fuel line that pumped fuel down the side of the hot engine and let it pool around an almost extinguished tire. I heard this right about the time my bottle ran dry. Next thing I heard as I was walking away in disgust were the tires exploding in the re-kindled blaze. Yep, it was a good one.
2. It's lack of heatsinkability renders it not so hot against solid combustibles like car carpet. It will knock the fire down but leave stuff smoldering to provide re-ignition sources.
3. Your car will be ruined. Save yourself and your stuff if safe to do so, but once you have hosed the car down with dry powder, it's junk. Very fine, very corrosive means death to all electronics.
I'm currently liking the Kidde fire-out foam extinguishers, not quite as much knockdown as a powder, but good on solids, good reflash protection. Ideally, have a dry powder to beat the blaze down and a foam to keep it there.
I also like Halon, still available, but buy them now, unlike the AW ban, it's not coming back due to EPA regulations.