How wide is too wide for mostly highway and city driving in a Silverado?

 

My mom has 305's on her truck and it rides awsome the only time her truck goes off road is in october durring the deer hunt and thats only for about a day or so everything else is highway. I like the look of something around a 285 or 275 wider then stock but not to wide.

 

A 285 is fine for a stock/not high lifted truck.
Thats about 11.5" width.
the 265's just look too narrow from the back.
and something like a 305 is heavy and will cause a big power loss..