The 33 mpg is for 55 mph.
Remember when the car was new the speed limit was 55 mph.:p Also yo may need new fuel filters, screen in the tank may need cleaning and you might need something run through the system to clean things like your fuel lines and injectors. I used some biocide followed by a tank of B100 and I'm averaging about 27 mpg around town in my 83 automatic and highway seems to get me about 30-32 mpg at 60-65 mph.