255/70/18??? bad idea?

hey guys, i found a pretty good deal on a set of like new bridgestone duellar a/t tires from a new jeep wrangler (the 4 doors) this is the stock tires that are comming of the jeep, do you think its a bad idea to put this set of tires on the titan? stock size is 265/70/18... it is about 1/2" shorter overall, and 1/4" on each side... will this make a huge diff on my speedo? and gas milage? am i just better off not bothering with these? thanks