Why is my MPG down on my F150?

I have a 2004 Ford F 150. When I bought it, it was around 16-17 MPG. But for the past few months it's been around 11.5 MPG. Why is this? My tires are properly inflated. How can I increase this?

When was the last tune up performed? That would include: oil & filter changes for engine & transmission, new plugs & wires, adjust timing ( if applicable ) etc etc… An improperly tuned engine, along with dirty lubricants, can result in lower fuel mileage

Tires definitely have something to do with it. Try having them checked, you might need new tires. Otherwise you should be able to go and have them adjusted correctly at a jiffylube or the like. What kind of oil are you putting in? Try going for a synthetic such as Mobile1, and also, what octane gas do you use? Try putting in the more expensive one, make sure it isn't diesel, unless the car runs on diesel fuel. Usually 92 and above are the higher octane available. Otherwise you may have to take the car in and have it looked over a little more closely than routine maintenance.

Compression differences due to age. An engine rebuild would raise the efficiency back to stock, only if you feel it is worth the investment of time and money.

Everybody missed an obvious gas waster. Dirty air filter.
Let me tell you about F Series trucks. I have had a dozen of them and have one now. The brake caliper are notorious for failing to release the rotor. They drag "just enough" that you will not hear or smell them but it will knock your MPG down. The problem is the sliding pins on the calipers. They rust and will not retreat or return to the released position. Rebuilt calipers come with a rubber boot that protects the pin (bolt). At least until road salt attacks it again. Pull one wheel at a time and make sure those calipers are releasing FULLY. They should slide on the pin.