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My '95 F150 is so reliable that I don't carry any extra tools or equipment. I carried a lot of stuff for years and often when I broke down, it was something I didn't have anyway. Probably the best thing to carry is a cell phone. The fine extinguisher someone mentioned also sounds like a good idea, but a quick disconnect for the negative battery terminal would also be good to help put out that electric fire--if nothing else, the appropriate sized wrench fastened under hood near the battery would help.
I also carry 2 hammers but you need to have Duct Tape and coat hanger wire for the really tough jobs. If these items can't fix it then it can't be fixed. :-)
Norm Wagner
1993 F-150
1995 Explorer
1996 MarkV111
All of the above plus I have body filler and activator. The stuff comes in handy when you have a truck down with a hole in the radiator. Just mix the stuff up hot and your ready to roll within minutes, as long as you have a jug of coolant in the box also. I have done this trick several times over the years. Strange things can happen on construction sites.