Have a 2002 F 150 4x4, 5.4, crew cab, 208,000 miles, automatic transmission..it starts and runs really good, accelerates good, but gets horrible fuel mileage, around 7 or 8 mpg, and I can smell the rich exhaust, but sometimes it smells normal. I do not have a lead foot, and I do not carry any weight around in the bed. I do have after market wheels but they are the same diameter as stock as just a little wider..truck has no lift. Only code I have is the P0136 which means the downstream 02 sensor is bad, but the downstream sensor I know does not affect fuel/air mixture. Upstream sensors are switching good and have proper voltage. Short term fuel trims are +-10%, long term varies but is not too high and does not get below -5%. New plugs, new fuel injector o-rings, new intake gaskets. Fuel pressure is 30 at idle and 0 at KOEO. vacuum is about 20 at idle. Cats do not appear to be plugged because of the good performance and they do not get red hot or anything. Anyone have any suggestions, this has got me stumped and I'm about to pull my hair out lol.

 

I have a 2000 F-150 5.4L 4x4 with 140K on the clock and I get 10mpg in town, but it is short trips within town only.
I checked mpg on a long freeway trip and got 18mpg
I have kept up the maintenance, spark plugs have 5k on them, I have replaced all 4 O2 sensors
what type trips do you take?
Do you have a 5w-20 oil in the engine?
I have to ask is the intake air filter clean?
at 208k, you should be on your 3rd set of spark plugs