Suby 2.5 in a lightweight coupe? I'll bet you could cruise at 75 and get 30-35 mpg. You should be able to get 400 miles of range and still have a reserve easily with that tank.
Around town with a lot of "brapping"? Not as much as interstate cruising, but I'd bet at least 25mpg on your worst day.
Beautiful tank fabrication, Carey!