if you have stock rear tires, leave the sprockets alone

if you are going down to a 21" tire you might want to go down 1 tooth on the rear

if you are going down to a 20" tire go up 1 tooth on the front

if you are going down to a 19" tire go up 1 tooth on the front and down 1 tooth on the rear

I took the 22" rear tires and 21" front tires off my 88 LT250R and ended up running 19" rear with 20" fronts. The absolute best thing you can do is buy a pair of G-Force extended rear hubs for a Banshee. This way you have a more common bolt pattern (4X115) and you can switch to 9" rear wheels so you have more choices in tires.