Depending on driving style, the $20 gotts mod
might help a teeny tiny bit, same goes for programming. Driving style reigns supreme on mileage.
The easiest way to boost mileage I found is to install one of those $15 vacuum gauges. It doesn't do anything for the vehicle but it'll teach you to drive more conservatively. You just try to keep the vacuum as high as possible by using less of the gas pedal.
Here is a good article to read,
Vacuum Gauge Usage For Gas Engines. It is dealing with RV motor homes but the numbers and concepts are the same. The driving style is the important part, the second part about other uses for the gauge will not apply to your Expedition.