I50 tridata speed reading is incorrect

I recently replaced an st60 tridata with the I50 and a new P120 low profile transducer. The the temp reading is correct but the speed and trip distance is not correct. Connection is seatalkng with a E127 and A9 on the network. The i50 speed is about 4 to 5 knots lower than the E127 SOG. How do I correct this. thanks

Ed Collins