Nice reply, Longshot!
I would not be surprised to see a fuel economy increase in the 50 mph range with slightly bigger tires. I hardly ever drive at that speed in Arizona with our 75 mph interstates and 65 mph country roads (everyone runs about 80 mph and 70 mph on each of those).
About the only time I'm at 50 is in the city, where the speed limit is 45.
(I'm keeping up with traffic).
- Jack