A 245/40/17 tire does have the same diameter as a 315/40/17 tire, but only on the inside of the tire where it mounts to the wheel (rim diameter). The outside diameter of the two tires will be different though. The first three numbers of the tire (ie. 245, 315, etc.) are the overall width of the tire, while the second two numbers (ie. 40 series) are the aspect ratio of the sidewall height to tire width. Check out the specs @ http://www.tirerack.com/tires/tiretech/general/size.jsp So, a 245/40/17 tire is 24.72" in diameter and a 315/40/17 tire is 26.92" in diameter. Over 2" in size.