Maybe it's still restricted? Check out dtr125.net for info on making it faster.
My 2007 DT did 65mph when it was all stock, then I earthed a wire behind the speedo and it rocketed (literally, in terms of performance gain) up to 76mph and that's 1 of the many restrictors! Removing them all and changing sprockets can result in daft top speeds of 100mph+
http://dtr125.net/tuningnewer.htm