Hmmmm? I used to get 21-22 MPG easy on the highway and 15-16 MPG in the city with both my 2001 and 2003 Sport Trac. No chips, no exhaust system, no special intakes, etc. Just a K&N drop in filter to replace the stock paper filter.
Save your money ! MPG has more to do with your driving habits and the environment that you drive in.
One add-on that will help you monitor your gas mileage is an Ultra-gauge or Scan-gauge that gives instantaneous MPG read outs, That will allow you to see exactly how your driving, the weather, terrain, and traffic effect your MPG.... You can only change your driving habits, but that is often good for 2-3+ MPG. Sometimes leaving at a different time, or taking a different route will avoid heavy traffic and give you better overall mileage even though the route might be slightly longer. Remember, when you are stopped at a traffic light, you are getting 0 MPG !
....Rich