From a mechanical standpoint, it would seem that 85 and above, is hard on vehicles that aren't meant to drive that fast for long periods. Over-revving, etc.
From a safety standpoint, it would seem that the faster you go, the worse the crash would be. That's literally, a no-brainer. Literally... And that's fine, as long as it only affects you.
From a mileage standpoint, it seems to depend more upon the vehicle. My half-ton Chevy truck gets worse mileage at 85MPH, where as my Mustang doesn't seem to notice it as much.
In the larger picture - I doubt posted speeds will change. If you want to drive slower, and do so safely (the RIGHT HAND LANE IS FOR SLOWER TRAFFIC) then go right ahead. If you want to drive faster, and are fine with brushes with the law, then go right ahead.
Just don't be a selfish @sshole and lose control and take out my family.