Yes, I think the aerodynamics are much worst than say a sedan.

The other piece, I believe, is engine speed. The 6 speed seems to shift to top gear rather quickly. At 60mph, I tach at less than 2000rpm and the engine is hardly working. At 80, tach is ~26-2700? Of course, going faster, but making more HP than really needed. I suppose the variable valve timing may have kicked too?

Some of the mercedes 7 sp trannys are really efficient in saving gas. Really tach low at cruising speed. So, If you want to drive economical - drive slower.