The fishhawk speed and gps speed may never be the same. You have to take into account the direction the water is moving along with the direction the boat is moving. Going into the water current will give you a faster fishhawk reading and a slower gps reading. Going with the current will give you a slower fishhawk speed and a faster gps speed. This is why speed at the ball is so important. Expierenced fishing can tell how fast the boat is going by the bend in their diver rod. You can calibrate the fishhawk speed to gps speed, but you should do this on an inland lake with no current.