I have 2000 Excursion also. Best milage stock on hiway under 65mph was 20.7. that was about the only long hiway trip I'v made. Combo Hiway city commute stock low 16mpg, installed AFE prodry stage 2. and MPG went up to low to mid 17. That was several tankfuls. Only done short trip city and hilly driving since 4in Exhaust and DP Turner. These fugures are all hand Calculated.
Stock Lie-O-meter was with in .5 mpg up or down. seams like with Chip Lie-O-Meter reads hi. One short tankful hotroding around it seemed the hiest.
My Daughter has same 2000 Ex. Diablo sport and K & N air intake. She's too lazy to hand Calculate, but claims 20-24 hiway by Lie-O-Meter.