In my Caddy, I get the same mileage (+/- 0.5MPG) at 70 as I do at 60. Mine may be an oddball, because of the very tall gearing (I'm under 2000RPM at 70MPH), but it doesn't seem to make any difference.
Unless the engine is totally inadequate (like most beercan economy cars), the A/C drag is almost unnoticeable. On my Caddy, my truck, & any of the wreckers (all with large gas or diesel engines), I can't hear it even when listening for it. On my Olds wagon, I can hear a slight change in RPM at idle.
Interestingly, the F-800's compressor (air brakes) makes the idle speed dip slightly (but noticeably) when it cuts in.
Our Civic gets its best mileage (far above Honda's estimates, actually: between 40 and 50 mpg coming through the plains states with no wind) in the 65-70 range.
