Simply put, every vehicle should have one.  I chose a larger A-B-C type unit because in addition to oil, gas and electrical fires, it is also possible to have organic materials ignite, like for example leaves against a hot catalytic converter.  If you saw the episode of the Sopranos where the SUV goes up in flames because the leaves underneath it caught on fire, you understand what I mean.  A plain B-C extinguisher isn't enough.

I had to get a little creative with this install.  I bolted to the cross-member behind the driver's seat a Jeep roll bar grab strap that had a plastic backing and two heavy Velcro straps that wrapped completely around  the Fire extinguisher. 

I bolted the bracket that came with the extinguisher to the cross-member in such a way that the fire extinguisher will not move laterally.

Then I placed the fire extinguisher in the "cradle" and closed the two Velcro straps on it.  The extinguisher doesn't move, rattle or shake, but is easy to whip out.

Users Manual