I have a 2005 F250 powerstroke with a 6.0 since I got the truck I've gotten great fuel(22 mpg interstate) mileage until now. My filter(stock) was dirty so I bought a new filter(airaide) to replace it I'm still getting the same bad fuel economy down to 15 mpg. What is wrong here?

Cold weather is responsible for part of the fall off. Using full-synthetic HDDO, transmission fluid, and gear oil will likely improve mileage by 8 to 10%.