re: OB motorheads- will supercharging a 6.0L V8 gas engine

It'll decrease the MPG. Due to: parasitic drag via the belt that runs it, plus, with engines, more air = more fuel = lower MPG. This is the reason that in real world driving the Ford EcoBoost is anything but economical. Granted, with turbos, you don't have the parasitic drag issue, but you do have the more air = more fuel = lower MPG issue. If people were capable of driving it without getting "into boost" it would be more economical than a V8 because it would essentially be a 3.5L, which is more economical than a 5.0, 5.3, 6.2l due to lower displacement. But, once you spool the turbos up, the air charge per liter is denser than it is in a larger NA motor and thus so must be the fuel charge, and as such, the amount of fuel being used more or less evens out.