Okay, "Dramatic" may have been a little bit of an over statement.  On the highway driving 60 to 65 mph on flat roads with little wind in my '85 Weekender 1.9L Automatic I can get 19 mpg.  Staying in the 65 to 70 mph range the mpg drops to 18.  After changing to a K & N air filter and Bosch Platinium +4's I was real impressed with the extra pep and made a road trip averaging 70 to 75 mph and then some (80+ felt cool for a couple minutes).  The result?  17 mpg.  Two mpg difference with a 10 to 20 mph speed increase could be dramatic then again maybe not.  The difference could be tires, wind, terrain, traffic, etc..

Difference in engine wear may be equally as "un-dramatic" -- the difference between getting 100K miles out of an engine and getting 99K.  There are too many variables to make a blanket statement about engine speed being safe or not.

I don't have enough in the bank to cover a new engine right now so I'll take it easy on my wheezing water boxer.

Chuck