My 90 w/ 454, turbo 400 and 3.73's gets 10mpg at highway no matter the speed.
My 87 w/ 350, 700R4, 4.10's 4" and 35's gets about 8mpg avg.
Both are full Silverados, 3 rows of seats, front and rear AC, cruise . . . both have 35 gallon tanks . . .
If you can squeeze close to 10 mpg out of it...