What kind of road conditions? How long at specified speed was needed to obtain the average? Traffic?
Too many variable my friend. While my GP may lockup at 38mph, here in MT that will not net me the best mileage due to the hills and mountain passes. Dropping my speed from 75mph to 70mph lowered my mpg, just from the decrease in inertia attacking the passes and rolling hills.
I'm guessing you used a scangauge? My dad bought one this last fall, and at 9mpg in the truck he is very motivated to increase that, but has been unable to just because he is already as the limit of his conditions for FE.
Don't take this as an insult or negative comment. Just some of us live in places that just don't allow for amazing FE.
Would be nice to hit 36mpg with a full size car, but its just not possible where I live and my driving conditions.