2004 ford f150 supercrew lariat 4x4 140,000 miles. I had a CEL on for about 2 months, it was because of a clogged CAT. At this point my MPG were 15.9 at tops and 14.8 if my son was driving the truck. I put new A/T tires on it and the MPG dropped to 14.8 tops and then slowly dropped to 14.4 over a couple of weeks time. I replaced both CATs and Both O2 sensors on the drivers side, after this MPG has dropped to 14 and wont budge. I thought after I replaced the CATs MPG would go up. I have had the truck for 6 months and have done 2 oil changes, radiator flush, air filter, plugs and COPs. I always use the 87 octane gas. is there something Im missing or is this average MPG?

 

How long ago did you do the cats? I believe the CPU needs 100+/- miles before the sensors compensate for the change.
Also did you unplug the battery for 30 min during/after the cat job to reset the CPU?
if not, I would unplug the battery and then put 100 miles on the truck and see if there are any changes

 

I did not unplug the battery, but I had the cats replaced a week ago then four days later went on a 400 mile road trip an it didnt move from 14 MPG. I erased the codes after the cat and o2 sensor replacement. the CEL didnt come on in that 400 miles. I was thinking every thing "reset" after 40 miles?