Ford Ranger - Sudden Decrease in Gas Mileage

1997 Ford Ranger XLT Mileage: 170000

I have had a consistent gas mileage in my truck of 300-350 miles per tank. Suddenly, the truck is only getting around 225-260 miles per tank. The difference is remarkable. Otherwise the truck seems to be running fine. The 'Check Engine' light is on, but so far I've ignored it, as the 'check engine' light has been on for the most part of the ten years I've owned this truck. HELP!