the problem is the pick up wheel at the left rear of the boat, it is too low in the water. it is slightly adjustable with a few screws you can loosen and move unit up. if you feel under boat the bottom of pick up wheel should not stick out past bottom of hull. it causes turbulance and wheel wont turn fast enough and you get a low reading. i took a file and filed off bottom untill flush with bottom of hull. my speedo read very slow and that fixed it, hopes this helps.