Before the plugs were changed three weeks ago (at the Ford dealership),the truck was getting about 15-16 mpg.
Now it gets 13 mpg.
Any thoughts or suggestions ?

Truck is a 2005 Supercrew,5 1/2 ft. bed,5.4 Triton w/ SIDO exhaust

 

Put the OLD plugs back in!

 

Clever code58 ha ha

My truck dropped from 16.5 to about 11. I have not changed plugs but I have seen this several times on this site. I went to 285/70/17s and got a k&n filter, that was the only change. Mine was at 13 for a few months but it dropped again the last few tanks. It seems to me that this is happening to a lot of trucks and may not be a result of a change.

 

Are you buying your gas from the same place each time? Might have more ethanol in it, which will affect milage. Checked tire pressure lately?

 

What else was done along with the plug change?

 

Was the Neg battery cable left off during these plug changes??
It might be that the ECM needs time to relearn?? ,just a thought

more plausible perhaps, is that a vacuum line was inadvertently knocked off during the plug change procedure....or some carbon in the cylinder got knock off during "the plug removal", and is under a valve of 2