Hey guys,
I got a 2011 fx2 SuperCab 5.0 with 3.73 gears and stock 20" rims running Pirelli Scorpion A/T 275/55/20's with 48k miles on the truck. I've had some rough idle issues, especially when cold. So I bought some motorcraft SP-519 plugs (oe plugs for the 5.0).

Recently went on a 720 mile round trip, installing the plugs when I arrived at my buddies house I was staying at. On the way there, I averaged 18.0mpg on the lie-o-meter (hand calculated 17.83). I changed out the plugs, noticing that number 7 cylinder had quite a bit of carbon build up on the plug.

On the way back, I kept the cruise control at 72 (same as the way there) and the truck was pulling 20.5-21.2 on the lie-o-meter. I was shocked but expected different numbers at the pump. hand calculated 20.8 mpg.. There are quite a few variables at play, but I've never seen the lie-o-meter up that high ever since owning the truck.

Also to note, my rough idle issues seem to have gone away for the time being, so to all you 5.0 owners, change those plugs!

 

Nice.... I am wondering if it was really a combination of needing new plugs and the Spark plug gap being off?

Did you check to see what the old plugs were gapped at?

 

Yeah they were all about .049 to .053, so not terrible.. but at .045 the truck smoothed out at idle... Its like having a new truck again!