ALL FOUR HAVE TO BE THE SAME SIZE. if you put different sizes on one axle it will mess up that axle's diff, and if you put different sizes front and rear it will mess up the center diff. so either way you have to make sure the tires outside diameter are all the same. you cant always go by the size cuz a goodyear 215/55R16 could be taller than a nitto 215/55R16. on www.tirerack.com they tell you the actual height of each tire so you know what you can and cant use. hope this helps