I'm currently running BFG A/T 285/70/17's on my 05 F150. It has a 3:73 LS rear end. My spare is the stock that came with the truck, I think its 265/75/17. If I ever get a flat will running the different size screw anything up? I plan on getting new set of tires next spring so may transfer best old one to spare if it would help. So far 55K on these tires and still have 7/32 left.

 

I doubt very much that your spare size is 265/75R17. More likely it is 265/70R17. If you get a flat, make sure the spare goes on the front and that you do not use 4wd. Otherwise you could tear things up.

 

Yeah you may be right, just didn't want to crawl underneath the truck.

 

Since you have the limited slip, the rear tires have to be the same size. If they are different sizes and you get a flat in the front then you can just swap it for the spare. If you get a flat in the back, then you will need to swap a good tire from the front to the flat in the back and put the spare up front.