text: I agree with arbiter's approach and, mostly, his answer.  But I'd use an alternator efficiency more like 50% and therefore get about 75 ml/hour of gasoline.  (0.02 US gallons/hour).\n\nFor my Prius cruising at 65 mph, getting 48 mpg (1.35 gallons per hour), that would reduce gas mileage by 1.5%.  Or from 48.0 mpg to 47.3 mpg.  Maybe a touch less because the gasoline engine only comes on during more efficient loads (i.e. it never idles or operates at 5%) so it does a bit better than 30% efficiency.  \n\nI know a really meticulous German guy who approaches 70 mpg in a diesel VW by using all the tricks - no lights, no fan use, keeping the junk out of the trunk, shutting down as he approaches stops, high-pressure tires, slow starts, etc.  But, man!, it is nerve-racking to drive with someone like that.\n\nFor a Hummer getting 14 mpg on the highway (with a tailwind!), it would drop to 13.94 mpg.  Pretty dang hard to notice.\n\nOver 100,000 (daytime) miles at an average speed of 30 mph, 0.02 gallons per hour adds up to 67 gallons.  For either car.\n\nOf course, one head-on collision by someone who didn't notice your car, and you'd wish you'd spent the $200 on the extra gasoline.\n\nInterestingly, if the Hummer averages 12 mpg over 100,000 miles, and gas is $3/gallon, it will cost $25,000 to keep it fueled up.  With which, one could have bought a Prius.  Or two Echos (which would seat 10!).  Hmmmmm.