I have an 09 f350 v10, came with the 275 60 18 crappy contitracs, I got some 2012 20" take off with 275 65 20's, I am thinking about 295 60 20's or 305 55 20's, will these kill my highway mileage much? will I notice much between the 18 and 20's? My last truck got better mileage on the hwy at higher speeds than lower doing this.

 

Usually larger sized tires will kill mileage but the trade off is the appearance. The rolling resistance, mass, etc come into play. The speedo can be less or in some cases more accurate.

 

Larger diameter tires will allow you to run lower rpms at a given speed, so in theory, you'd get better highway mileage. However, the extra weight and increased rolling resistance of bigger/wider tires offset the theoretical gain.