Okay, so the scenario is a passenger car, traveling on the highway. As a general rule, I say that increased weight (mass) will have a beneficial effect on MPG (IF tire pressure is increased so that rolling resistance is reduced to compensate for the increased weight), ONCE the vehicle is up to speed. Essentially, (again, assuming tire pressure is increased to compensate for rolling resistance increases from weight), if a helicopter could swoop in and place a 500 lb weight into the trunk of a car already traveling 75 mph on flat ground......that weight could actually INCREASE mpg if the vehicle remained at precisely 75 mph.....over the course of 200 miles. But.....someone recently claimed that this increased weight, thus inertia, also would reduce the vehicle's energy consumption in order to overcome atmospheric drag....thus remain at 75mph. I said it would not have any affect on drag, and that the wind drag would remain a drain on efficiency, no matter how much the vehicle's mass / inertia was. I am certain that I'm right....otherwise we'd all put 400 kg weights in our trunk when taking road trips. However, I can't seem to conclusively prove this using math or better examples. Increased inertia requires energy as an input...which is used up over time for various reasons (hills, bumps, etc.).....but what about a constant wind drag? Seems to me that this initial inertia would be quickly used....and then replenished constantly in direct proportion with the atmospheric drag. Thus.....zero net benefit over time. That make any sense??