For all you Gadgetmen out there, from my experience ALL Fords in the Truck Series from '98-2002 generally get double their mileage, and enduringly.
Once, I had one that was so good, it would throw a P-0435 code once a tank (Cat below efficiency) that would occur. I dealt with that by resetting the codes, but could never get rid of it.
That was an 02 Expedition, 4.6L. It was mine (now it's Debbie's wherever she's hiding in shame) and I didn't mind resetting it myself but you might want to warn your customers! I didn't mind because I went from around 7 to more than 16 in town!
Love you guys!
Ron