it may be a while before i put on the 39T sprocket. with over 700 miles on bike the engine has become substantially more compliant, and cruises easily at 45mph. i've had several highway runs at 55 and everything seems fine at that speed as well (although the engine is obviously hurried). all speeds are indicated, btw. as the bike does see the occasional deep sand, mud, dirt and gravel, i'm trying to keep enough torque to the rear wheel to keep things moving. given steve's reports as well, it would appear as if the 42T rear is about optimal and a 39/40T sprocket may be too much. as soon as i get my ass in gear i will try out the 39T, but i'm really not feeling the need at this moment.