I installed BFG 305/65/18's on my 4x4 screw. Should I also change the spare tire size? If so, will a bigger tire fit in the spare tire compartment under the bed?

 

Ide say no need. There isnt much difference in height. Most cars dont have a full size spare anyway so now you are no different. I know that if I had to change out one of my 305s ide be crying about the price of the tire not that the spare isnt the full size.

 

pics man.....

 

If you have a Limited slip differential then you should make it the same size spare. Otherwise the clutch packs in the differential will fry themselves out. You could get yourself like a couple miles at low speeds and probably be fine, but no highway driving or anything like that. Also you wont be able to use 4x4 if you dont get the same size spare. If you have the money i would get the same size. And i think it will fit under there on a superduty. I could have sworn some guy had a SD with a 315 spare, i know the guy had oversize tires, i dont know how big but it was bigger than stock and he had it under as his spare.