Theoretically yes. But like introductory physics class you are talking about the mythical frictionless surface, non-existent wind resistance, etc. An 8 cylinder engine has 30%, approximately, more parts moving across surfaces that do have a certain amount of friction. Combine that with combustion inefficiencies, and you will probably come up with most of the differences in mpg's, between an I-6 & V8.Forgive me for not using the absolute exact technical terms. My understanding is that fuel consumption is purely related to the required horsepower needed to move a vehicle at a certain speed. That requirement will come primarily from the CX (aerodynamic coefficient) and the weight.
Under this premise, the fuel consumption of the V8 and the I6 should be very similar when traveling at the same exact constant speed. There are other factors of course, like wind, incline or tire size, but I am going to assume that those are equivalent between your case and mine.