I have a 2002 F250 5.4 4x4. The period of Feb 2008 through dec 2008 I got just over 10mpg (say in the 10.4 average range.) Since then it's gotten worse to where in the last two months I haven't averaged over 9.4mph. In Feb I put on tires, changed the fuel filter and greased the caliper pins. I have put 3,300 in the last 13 months on the truck. I did o2 sensors either the year before or earlier that year, I lost my records in a hard drive crash. They have less than 10,000 miles on them in any case. Oil was changed around December, along with new filter and I have about 700 miles on them both now. Swapped my K&N filter with a new "factory oiled" filter in Jan, and cleaned the MAF. Tires are 10% under max pressure, o2s should be good for another 3,000 miles minimum, I changed fueling stations to see if it was a gas issue (was citgo last two tanks were sunoco), all fluids look good, I just started using the AC again last week, no mods, just washing and waxing in the last year. I've looked at some of the parameters and it seems the engine is running a tad on the lean side when parked and not really rich at acceleration or cruising. I am going to clean the MAF, but air flow looks to be metering correctly. Anything else to look at on a 7 year old 74k mile engine?

 

Transmission, or any other part of the drivetrain creating some sort of drag?

 

Nothing has changed in the last 4 months to my knowledge. I just changed my tune this week. I am getting gas in a couple of days, so I'll see if it's changed anything. However, this tune should run a slight bit richer. Funds have been too tight to do much.

 

I heard that! Anything changed the areodynamics; missing tailgate, new roof rack, removed air dam, ect...ect?

 

Nope, I've waxed a little more often, but the bed cover and tailgate are in their normal positions, smooth topped, the works.

 

What was the condition of the old tires before you replaced them? Are the new tires larger/wider than the old tires? Did you go to a more aggressive thread pattern?

 

The mileage comparison is with the new tires only. They are Toyo 315 Mts, I lost just less than half an mpg with the change but I was averaging in the 10.4 range with the toyos and 10.8 with the warn out BFGs.

I really hope my mileage isn't fading with 3,300 miles on these tires. Speedo is pretty correct, 52.5mph on the cluster is 55mph on my GPS and 55mph on the cluster is 60mph on GPS. I've had to use a tire size of 37.5 in the computer to get it this close, low speed is under the cluster's reading.

 

Maybe the replacement of the K&N filter with the factory oiled filter it causing the lower MPG's. Try disconnecting the battery to initiate your PCM to re-learn with the new factory oiled filter.

 

The ECM has lost power several times, I have a gizmo that disconnects the battery if it falls below 11.8v. It can do that after 5 or 6 days of sitting.

I am thinking it's a fan clutch problem. My chiltons is as useful as a chiltons, I can move the fan blades cold, but they don't move freely. It's much stiffer after it's had a little heat. However, the belt squeals during most shifts.

 

I guess it is possible for a faulty fan clutch to cause enough resistance to lower your MPG's. If so, then it's a relatively inexpensive and easy repair.