Welcome to the forum. Average mpg is such a vague term. It probably depends more on your life styles than the vehicle. During the winter, I could get way below 10. But then again, living in snowy climate, idling time, frequent stops, short commute all contribute to my very low average mpg. I don't think that number is that meaningful. Instead, try to measure what your mpg is for a controlled situation. If you could drive relatively long distance at about 55 mph you should be able to get over 17. If you are still getting 13 in ideal circumstance like that and your truck is stock, I would think something is wrong.