I usually get 39-43 mpg on the highway at 75-80 miles per hour (my < 40 mpg tanks are due to city driving). The car seems to do better when it's warm (day) than cool (night), a difference of around 15-20 degrees.

That's without cruise-control. Also I take care to coast alot when slowing down to leave the highway, which definitely helps make up for all the gas lost from accelerating onto it.