RKB

I have used a gps against my speedo on my 2010 Speedster 150. The speedo is off by 6 mph. Has anyone else seen this? I expected probably 3 mph. Also, the best performance with 2 people on board I have seen is 46 mph. Does this sound correct? Its not the 255 hsp motor but I was guessing at about 50 mph for this boat. Please give me your feedback. Thanks RKB