You'll generally see EPA estimates in the range of:
* City: 16-18 mpg
* Highway: 24-26 mpg
However, actual mileage will depend on driving habits, vehicle condition, and other factors. The quoted figures are just estimates. To get a more precise idea of the gas mileage for a specific 1997 Buick Park Avenue, you'd need to consult the owner's manual for that particular vehicle or check online resources that provide user-reported gas mileage data (like fueleconomy.gov, although data for older vehicles may be limited).