They will make a negligible difference. The width of the tire affects the mileage way more than the weight of the rim. Consider that yes, a heavier wheel/tire takes mnore energy to get moving, but once it's moving it wants to stay in motion due to the increased kinetic energy. Wind resistance from the width is much more of an issue. If you run the same size tire on a Hummer wheel vs an aluminum wheel, I'd bet that mileage would be virutally the same.
This is kind of apples and oranges, but I see similar issues to this on a VW TDI forum. Guys like to put on 17" wheels with wide tires and it knocks about 2-3 MPG off their averages. But these tires are much wider than stock, and the cars get 50+ MPG, so the percentage of mileage loss is about 5% or so. Considering that you probably get 15MPG or so, that means that even if you lost the same percentage as they do, you'd lose .75 MPG. You could get that back by mounting some tires with a halfway efficient tread pattern.