Here are some other suggestions: Better MPG

I bought a new garmin gps that has eco challenge on it... that kind of helps you drive better to get more out of your tankfull.... it also will show you how much you paid in fuel for the trip you just took... its fun to try to get the most out of everything you got!

i actually got better mileage in town in my 6.9 than on the highway. it does sound poor though how old are your injectors?

A lot is in how you drive it. I'm running completely stock. Gettin about 16 in town, and 20 to 22 on highway. That's if I don't punch it to much and my hubs are not locked in. Here lately I've been leaving my hubs locked in due to ice and snow. So I'm getting roughly 8-9 city and 14-15 highway.

I read if you wait to long to change oil you loose the anti foam agent in your oil and then the injectors don't work right.