With a group of 9 boats, we ventured out from our home ports in Harrison Township (Lake St. Clair) to Lexington (Lake Huron). It was 66 miles each way.
The Vista performed great and all seas greeted us with a calm nature (somewhat unusual for the Great Lakes!). I made it to and from on 3/4 tank of gas so Im guessing 80 gallons for 132 miles = somewhere between 1.6-1.7mpg or around 16gph. I cruised up the St Clair River around 3200rpms/25-27mph. On the way home, going with the current of the river, I cruised at 3200rpm and 30-32mph.
If you've never experienced Lake Huron, its a treat. In spots 30-40' deep, clear all the way to the bottom!! Pretty much uninhabited. The Lexington marina was excellent right next to two public super clean beaches that never get too busy.
All in all, cant wait to make the trip next year! We've been to Lake Erie (PIB, Cedar Point, etc) many times but never to Lake Huron.
Oh, on our dock there were three 268 Vistas, not with our group. I told two of the owners about this site
One of the guys was telling me his 268 Vista with the 350 burns 7gph at WOT and weighs 9500lbs 
I didnt have the heart to tell him reality, as we were enjoying a few adult root beers and didnt want to come across as a mr. knowitall...lol