tuning your xv123 is easy first thing to do is replace the front sprocet from 14 tooth to 13 and drop the rear sprocet as low as 40 teeth instead of the 47 standard .Then change the main jet to 87.5 instead of 77.5 this will give you a new top speed of 65 MPH .But most of all you will not get blowen back eny less than 50mph in very strong head winds. And 60 MPH in average head winds . and the bike will cruse at 65mph no problem and i have had 80mph with wind behind me and i am over 6 foot and 13 stone