I changed my spark plugs last week at 37k miles and my mpg has dropped considerably since. The plugs are the OEM NGK's and I checked the gap on each one before installing. The only thing I didn't do was use a torque wrench to get them tightened exactly right.
Anyway, my mpg has dropped from 17-18 to 14-15 (about 3 mpg) according to the evic. I don't understand it, the truck runs and sounds normal but it just uses more gas. I was hoping it may return to normal after a few hundered miles but it hasn't. Any ideas??

 

I'd make sure you don't have a loose one and that the coil packs all got seated. Seems odd that changing the plugs would do that.