I was averaging 15.5+mpg. Latley I've been getting 13 to 14's. My driving style has not changed. It has been colder here though. I dont really let my truck idle any more. It has 54k on it. I havent done the plugs yet. I also haven't check my tire psi. What has changed and since i've seen the decrease is I went from a straight pipe to a magnaflow straight through muffler. Could this be the cause. It didn't take start decreasing right away though. My main suspicion is either my tire pressure and my spark plugs. Any other ideas?