I have 2011 F150 V8 5.0. currently, it has 124K and climbing within the last year I have seen my fuel economy on the high way go from 18.5/19mpg to 14mpg.

New sparkplugs at 100K, K&N air filter.
I use full synthetic oil.
the last 4 tanks of fuel I have put the Autozone Full fule system cleaner in (yet to see and improvement)

anything I can do to bring this MPG back up??

 

Is there a lift on it? What exhaust? What size and weight tires? Oversize or heavier tires are known MPG killers. What tunes, Gott boxes or other mods? Did you take off the air dam? Have you cleaned the MAF? I would do that before playing with fuel additives. My 2010 (built in 09) 5.4 FX-4 3.73 with about 110k stills gets the same MPG that it always has (18++ on the road at 75 MPH). NO mods to the truck or the engine except a hard tonneau cover. Original plugs and COPs and always MC oil and filters. No additives, tunes or any other monkey business. Stock 20" wheels and OE size tires and running at the recommended pressure.

We get they same complaint several times a week in this forum. My questions and reply needs to be made a sticky.

 

stock, nothing has been changed no tunes or lifts.
I did put a new brand of tire. went from wranglers to BFGoodgrid.

Only other thing would be that i instaled new belts (Fan/AC) and new AC compressor unit but i was having issues with the MPG before then

 

When did you change the tires and what wranglers to what bfgoodrich? Lots of options amongst those two tires (I am assuming k02's or the BF).

 

Sorry, I was incorrect on the replacement.

Went from Goodyear wrangler Duratec 116S to Firestone Destination A/T 114T

Both sized 275/65R18.

Here is what my spark plugs and throttle body looked like when I pulled them today.

I also Gaped the plugs to 0.045, they were grater the 0.050 (manual gives a range of .41-.47) and cleaned everything.

 

Install new plugs. Those look a little rough.

 

Stop using the AutoZone snake-oil fix. It is pretty pointless. run a few tanks of premium, ethanol-free gas and you'll be better off.

K&N are marketing hype and don't flow better than the OE pleated paper filter. But I don't think that is the issue, overall.

Have you reset the lie-o-meter? And have you hand calculated the mileage and compared?

 

Clean the MAF sensor, ditch the K&N filter and replace it with a Motorcraft OEM air filter ...
.

• CFM exceeds engine requirements/needs. *UNLESS über engine modifications are done. e.g. cylinder head and/or cams.
• good for 30,000+ miles (depending upon conditions).
• dispose and replace maintenance.
• any increase in air flow from after-market filters comes at the expense of filtration.
• no contaminating the MAF sensor or throttle body with oil. *Which reduces performance and MPG.

 

I cleaned them up and gaped them, they were new at 100K there is now 123k.

 

the K&N filter has been in the truck for 100K miles I don't think that this is the issue.