Highway mpg while towing is the only thing I truly care about with my truck. The only reason I care is for range estimation, which is extremely important when you only have a 33 gallon fuel tank. That is my only gripe with this truck.
Doing this test off and on over the last year and a half, I have found no power or mpg benefit to running 89 or 91.
Doing this test under city driving would be inconclusive, at least in my situation. The city mpg is so erratic with mine. It can range anywhere from 9 to 12 on any given tank, though I've never had a tank that was 100 percent city, meaning no freeway driving at all. The biggest factor seems to be time sitting idle and there is no way to compare this from one tank to the next.