Since other more reliable and knowledgeable sources seem reluctant to respond, I'll take a shot at it.

As related above, I use halon or clean agent halon which is what replaced halon after it was discovered that halon apparently causes ozone depletion. Halon is the same gas used to protect computer rooms. The benefit of halon is that it does not leave behind a caustic residue like dry powder extinguishers which will cause havoc to a car interior. The lack of powder residue is helpful if dealing with a small fire. The halon extinguisher is bolted to the front of the drivers seat where I can reach it while belted in the seat.

That said, I keep a standard 5lb ABC dry powder extinguisher in the trunk as well as a back up. I like the knock down power of the ABC, but there will be a substantial mess to clean up.

I believe the the best compromise between halon and dry chemical is CO2 which is what sits in my garage. Unfortunately, the CO2 extinguishers need to be rather large to be effective and are thus not favored for in-car installation.

YMMV.

Craig.