I like gearing down the early 900 Sportsters and K Models with 20, 19, 18, 17 . . . (and you can get really crazy with a 16 tooth transmission sprocket). It's fun to race around town and crank it up. I set my chain length for a 19, so I can run a 17,18 or a 20 without adding links, etc., and keep my rear wheel in a good "adjustment zone" for my range on the swing-arm slots. I keep extra chains of different lengths to run the 16 or a 22, 23 or 24 tooth. My road bike runs the best with a 22T . . . anything much higher just lugs the motor, loses power, and you run the risk of low oil flow, scored pistons, etc. I've found these ironheads need some revs to survive. My 1961 XLCH runs a 21 most of the time. I like to add an 18T now and then and she really rips ! You start power shiftin' with that 18, and the front wheel will pull up. Don't miss a gear, 'cause it grabs rpms real quick. With the XLR straight pipes, PB cams, XLR heads and other goodies, it's not long before the whole town just wants you to shut that thing off . . . and go home. Quietly.