I have a 2004 6.0 CCSB with a 4” lift on 315/75r16s and hand calculated I am looking at 13-13.5 all year long. No matter if it’s highway or city, summer or winter fuel. The truck is bulletproofed and has just about every popular 6.0 mod. Straight pipe exhaust and all that. Is this what everyone else is seeing or should I start looking for a problem?
With tires like that a a lift, you're probably in the right ballpark. Sadly, you're getting better mileage than I am currently. Not sure what it is, but my rig HATES winter fuel. I'm lucky if I get 13 in the winter. lol However, I WAS getting 16 in the summer as an average. Then, I installed a PHP FICM tune and mileage bumped by about 2 MPG...3 in some cases. So, that may be route you'll want to look into. I would also make sure you've got decent fuel pressure. From what I understand, lower pressures create larger droplets which don't burn as easily. Probably miniscule, but something to consider. Other than that, I've found my biggest improvement came from my right foot, the FICM tune, and making sure everything was running smoothly.
2004 Excursion 6.0
New HPOP/HPO system o-rings; Updated standpipes and dummy plugs
EGR delete; Rebuilt oil cooler; CCV collection mod
VR Black Diamond head gaskets and ARP studs
Oil and coolant bypass filters; Shell EC-1 coolant
Rebuilt turbo; 250 amp alt; 6000 watts of brain scrambling
One giant 240 pound English Mastiff riding shotgun!