Maybe he was repeating the EPA-Government Fuel Economy party line? If you shows fuel economy decreasing from around 30 mpg at 55 mph to around 24 mpg at 75 mph - a decrease of 1.5 mpg for each 5 mph incremental increase in speed. They also include the caveat -"While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds)...."
I don't see any way an expert could make a statement for every possible car, so maybe he was just picking the most credible source (at least in his opinion) he could find. My Expedition comes close to agreeing with this particular estimate. As best I can tell my Expedition will average close to 20 at a steady state 55. At 75 that drops to around 15 - a 1.25 mpg drop for each 5 mph increment above 55. I think my old F150 is about the same. I rarely drive it on the highway any more. But back when the speed limits were a uniform 55, and I drove around 60, I could average 20 mpg on a trip from my farm to Raleigh and back (150 miles). Since they raised the speed limits to 65 or 70 on the major parts of the trip, the F150 won't average 15.
Ed
