I don't drive much in wide-open spaces out West, but here in my region (Midwest) one can rarely drive a CONSISTENT 75+mph anymore. There are always clusters of vehicles driving slower, so real-world mpg penalty of trying to drive that fast is greater (from repeated accelerations vs steady speed) and real-world benefit is less.
On same highway trip between 2 big Midwest cities I've noticed no big net time savings trying to drive much over posted speed limits. The same cars you just passed a few miles back catch right up to you again as you reach the next rolling traffic jam (like big trucks matching speeds in both lanes
). So these days I just set the cruise at the posted speed limit & save (most) of the aggravation of fighting the slower traffic- and watching for speed traps.
BTW- on some length trips the better mpg's of driving slower & smoother can actually SAVE net time. On one of my mid-length (~350mi) routes I can drive ~65mph ave & easily make it straight through on one tank. Trying to drive 75+ knocks down the mpg's by ~20%(+) and forces a pit stop for gas. As any NASCAR fan knows, sometimes conserving fuel to avoid another pit stop can win you the checkered flag