I'll run 13.5, but don't see the point. The beauty of mod buggy is that it's run what you brung. No need to motor up or down for our track as a 17.5 is capable of getting everything done. IF we start traveling, then it's something to look into, but there aren't many of us that do go elsewhere.

That said, my 13.5 was A LOT faster than my 9.5