What is your actually mileage? Gas gauges are are hit or miss when it comes to being accurate.

Things I'd check:
dragging brakes, air filter, tire pressures and a complete diagonistic scan. The scan will tell you if you're staying in open loop for some reason (and your milage will SUCK), or if your timing is retatding because of a bad knock sensor or below grade fuel.

Worst comes to worst, try slowing down. I get around 25 mpg between 68 to 72 mph, 22 mpg at 75 to 80 mph and almost 28 mpg at 65 mph with cruise on. And this is with an auto trans too.