I use to carry a 5lb CO2 it was strapped to the front edge of my seat with velcro, easy to grab as your exiting the vehicle.. A couple years ago I had my daily driver burn pretty severely, a large shop style 10 lb Halon did not put it out.... I was told by the firemen that it's not uncommon for Halon to fail on a fire out in the open, it's very effective in an enclosed area but with a moderate breeze outside it dissipates quickly & as long as a part of the fire still receives oxygen it will continue to burn & reflash the areas that you put out.... They say the chemical powder is better in spite of the mess it makes...

Pretty in depth discussion in this link...
http://garagejournal.com/forum/showthread.php?t=163400&showall=1

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