Weird, i get more than that in my 90 legacy wagon AWD that has 135k+ miles.
At 78mph (the speed the car travels at 3k rpm) Im getting 22.5 mpg highway.
Usually I fill the tank brimming and dont fill up again until 260-305 when its around E.
Ive always been using chevron 91. My ignition system is currently stock, but the parts to upgrade it are slowly starting to pile up. It will be very soon when I hope to improve my mileage, or at least improve performance under hard accelleration.