Well I can tell you what I did. Base was 1987 745 Turbo with a AW71 all stock.
Best I could pull was 26.5 mpg at 65 mph and at 70 mph I got in the 24.5 range in fuel mileage. I lowered it and removed the roof rack to cut down on the air turbulence over and under the car. I then swapped out the AW71 for a M46 manual and swapped out the 3.91 rear for a 3.31 geared unit. I also swapped out the 15 inch Draco's for a set of 16 inch Hydra's. The car also has a Saab APC Boost control unit installed that gives me 14 psi at the touch of the throttle. The air box was opened up at the inlet where I removed a restrictive horn or cone in the tube going into the fender intake. I also went under the car and made sure the exhaust system was tucked up nice and tight. Finally I filled the Engine with Mobil 1, the transmission with Redline D4ATF, and the rear with Redline 75W90 synthetic oils.
The result is I can cruise at 70 mph at the old 65 mph rpms. The car runs about 2450 rpms at 70 mph. At 65 mph it is running in the 2300 rpm range.
The bonus is I now get 28 mpg at 70 mph and at 65 mph I run in the high 29's to low 30's. I added about 1 to 1.5 mpg to my around town mileage.
Due to the problem of the existing exhaust scrapping on driveways and speed bumps I decided not to go to a 2.5 inch system. A 2.5 would help add 1/2 to maybe one more mpg to my highway mileage.
To add, weight is a major factor in decreasing mpg. So go through the car and pull out everything you don't need from the interior and trunk. Also if you need to lose a few pounds it may be a good time to start a diet for you and the car. Also make sure you have a clean airfilter, fuel filter, and new plugs.
Make sure you keep your car aligned because it will save your tires as well as fuel.