PerfectPass problem in 04 Centurion

I have an 04 Cyclone with PerfectPass and up until now I have not had any problems with it. But for some reason now the speed is completely off. When it says we are going around 23, we are doing more like 30. For the end of last season I had to turn it down to about 15 and I was just guessing that was how fast we ride at. What could be making the speed so off? Any input is appreciated.

Thanks