Actually the operative word should be taller, rather than bigger, or larger circumfrence. The65/15 will have a circumfrence of 80.1 inch, while the 55/16 will have a circumfrence of 78.2 inches. Yeah, I hadn't paid attention to the 15/16 differential earlier, and didn't fully understand what you were asking. Hope that helps. The shorter tire, the 55/16 will help alleviate some of the speedometer error on the X, by 1.4mph compared to the other taller tire.