Cannondaleriders

If you go from the stock tire size down to an 18" MX tire do you need to change the sproket combo to keep the same power? Or is the gearing on the Cannibal failry universal?

i went down to a 38 in the rear, seemed about equal, now im going for more low end so im gonna run the 13x42 again, that gave a lotta pick up in a short distance, which is what i need, another combo which is very very good and i reccomend most would be the 14x39 get yourself a new front bigger countershaft sprocket and it'll save your chain buffer a lot longer life