Your 1A10BC fire extinguisher sounds good for auto use to me - if you intend to knock down a small engine fire or suspress fire while removing occupants to safety. Here's another link to some good advice on purchasing fire extinguishers - http://www.firerescue1.com/fire-products...extinguishers/.

Practice is essential. I make it a habit to buy new fire extinguishers approx. on the same schedule as new household smoke alarms (~5 years), and when I have a couple spares the wife, kids and I hold a short PASS party in the backyard. CERT also has regular training under FD supervision that got me started - before CERT I had never used a live fire extinguisher. Its darn simple, but you want to know how to aim the extinguisher, and what size fire you can expect to put out. Check with your nearby fire station and see if they can help you out - I think most fire stations have a supply of nearly exhausted fire extinguishers to teach with.

Whatever fire extinguisher you purchase for auto use, keep in mind its probably going to be a compromise between size and what type of fire it works best on - there may be better materials for engine fires, but they can be more expensive, and seldom if ever get used. A 1A10BC in your trunk on the other hand can help with alot of different fires, and is pretty cost effective, particularly a rechargeable one.

And remember to bolt it down using a factory attachment if its in the main compartment of your car - extinguishers make nasty projectiles in high speed accidents.