Speed

Hi,
I am getting different speed readings from the OEM speedometer and my fish finder (that has the wheel mounted on the transom). The OEM said 37 and the fishfinder said 30.
Does anyone know how to tell which reading is correct?
Thanks
Ron

[boats.net]

GPS is the only way to get an fairly accurate reading. The Speedo using a pressure system and fish finder uses a paddle wheel, no way of knowing which one is right.