standard gearing is 17x38 <--(uncorrected ktm mis-print) stated but in reality 16x38. <--- I always get confused on this part.(counter only).
I have changed my rear to a 41. wheeie machine is here. chain fits fine just flip the adjuster blocks 180.
40 tooth rear is prolly the most popular size.
to check your counter is easy, simply remove the (3) 8mm cover bolts and the tooth count is the last 2 digits in the part #.