As others have said, leaving the hood down is the best advice. Open, it's a perfect chimney. But that advice may be counter-intuitive, because what what you'll likely see is a lot of smoke, not flames. The temptation is to open the hood and find out what's going on. Then, poof!

For fighting a vehicle fire? From the few incidents I've seen, no fire extinguisher is big enough. A 4-pounder is the "entry level" size, and they're not handy to schlep around in a passenger vehicle. Better to spend the money on a more robust insurance package and walk away.

The nightmare scenario is coming across a wreck on fire with people trapped inside. A small fire extinguisher and rescue tools may help, but the odds are against you. I vaguely recall that one of our members faced this some years back. May heaven/fate preserve us from such a no-win scenario.