Running ac off of generator while driving.

Assuming 6.5 mpg and 65 mph, that's a total of 10 gallons of fuel burned per hour. The generator, running the roof A/C, burns about 0.5 gal per hour. Drops my overall mpg from 6.5 to 6.2

Your numbers may vary a bit, but the difference in fuel burned is negligible for the comfort provided.

MrWizard wrote:

as far as gas milage for the drive engine
it has NO effect
but since it does use fuel for the genny
your next fill up means more fuel from the pump to the tank
but this is NOT engine fuel mileage, this is additional overhead fuel use by the genny for 'personal comfort'

we get people on here, saying i used my genny for the roof top A/C and mileage went down,
NO your engine mileage did not change

You are of course correct, my "engine" mileage doesn't change. But my "motorhome" mileage does. ;) The easiest way to quantify the fuel consumption and comfort while driving WITH the generator/AC vs WITHOUT the generator/AC is to compare apples to apples; i.e. mpg to mpg.