Id say go with at least 18's because there is so much deflection with all that side wall, you start to get wheel walk, and driving more than 70mph can get hary if the tires are big enough, and if one goes down on the street the truck wont lean as much to the low tire side. All in all makes things much safer.

 

Its a 35" tire on a 16" rim, the '75' part is the ratio of side wall to width, so with a taller rim you get a taller tire.