This article and the results parallel my own experience with my 400HP C6 manual Z51, bought in 2005.
My average interstate mileage at 65mph is about 28mpg, and around town about 21mpg.
I'm sure at 35mph it would easily get in the 30's for fuel economy.
The joke in the family is that the Vette is our "economy" car!
My 300hp Lexus GS400 averages about 26mpg on the interstate at 70mph, 24mpg with A/C on at that speed.
Cheers!
Doc
PS: The fuel "savings" for me to convert to a 40mpg car would only be $100 a month. The fun in the vette is worth that and more...