I had one and put lots of mod $$ into it. The only thing that gave me any improvement was a K&N intake. Now we're not talking a huge improvement, maybe 2 mpg, if I kept my foot out of it. Opening up the exhaust did nothing for the mpg. Lifting it and putting wider tires on it killed the mpg. So, if you wan't to see your mpg number go up a little I would say an air intake but shop around because you won't save enough to pay for it. With all this said though, I got rid of my 01 Excursion because of the manifold stud issue. I missed it so much I just picked up a 250 with another V10. Before making the decision I penciled out the cost of a trip figuring diesel prices on the west coast compared to gas prices. Even though you may get better mpg with the diesel I believe you'll spend less overall staying with gas.