i think any gasser towing an enclosed will get 9mpg.

i'm hauling an 18' open with a tire rack and a miata on the deck between 72-75mph on pretty flat land in 4th and get 11mpg. this is assuming there's not much wind though, which hardly ever happens in the Kansas/Nebraska/Iowa region i mostly tow in. 2200-2500rpm range in 4th at 72-75mph seems to be the sweet spot for revs vs wind resistance.