I also have 18" wheels (18x9). However I have the slightly smaller diameter of the 275/65/18 than ksarnecki's truck. Mine is a 2014 Pentastar with 3.55 gearing.
I did loose about 1-2 MPG on the freeway/highway, but no difference in city driving. The difference came from the added rolling resistance of my Cooper Discoverer AT3's as my wheel/tire weight is very similar to stock since I went with P-rated tires. Truck gets 17 city/20 mixed driving/23-24 freeway (depending on how fast I drive)
ksarnecki's experience/mileage is almost a mirror image to mine.