1998 Buick Century

Recently replaced plugs, wires, air filter, and fuel filter on my wife's 98 century. She still has a problem with gas mileage. It does fine when at 3/4 to full. Anything less than 3/4 and it needs to refilled within 36 hours. She only uses the car for work delivering newspapers in the wee hours of morning. If it is on half a tank when she starts its almost empty when she finishes. Paper route is about 60 miles long from start to home.