We have found our best racing set up is 18x10x8 rear and 20x6x10 in the front. Of course it really depends on what you want to do, I wouldn't say this would be a good set up for trail riding but works well for us at the MX track. Keep in mind the last number in each is rim size so you have 9 in in the rear not 8 like us.

 

i run 20s in the back and i ride xc terrain and do fine, some areas are a little rough being kinda low but its a good balanced size, the 22's work good in trails also but there just to tall for my liking, 22 or 21s in front also would be fine.