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Four.
Start with the roughly 33KWh/gal LHV (chemical energy) of gasoline.
A gas engine is about 25% efficient (ymmv, drastically) at turning chemical energy into torque, so you've got about 8 KWh of shaft work.
An alternator is about 50% efficient (ymmv, but not too drastically) at turning torque into electricity, so you're left with about 4KWh per gallon. I figure for a typical 200W econobox with the lights off, that's about .05GPH. Turn on the headlights, and it's about .10 GPH.
A typical hybrid is on the order of twice as efficient at generating 12VDC.
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