Yes, this is a VERY common issue - probably ubiquitous. For unknown reasons virtually all VTX bikes and most Honda bikes are off between 5-15% on the speedometer. More specifically, the speedo reads 5-15% high. Yours is the worst I've ever heard of (10/55 = 18% high). This can be fixed using something called a "speedohealer". Search the interwebs and this forum for more info. However, be aware that fixing the speedometer using a speedohealer will cause the odometer to be off. Most VTX owners just recognize that this issue occurs, do the math in their heads to know roughly at what mph they are riding, and live with it.