How can I calibrate the speedometer ? I did some comparing speeds with a hand held GPS and the pitot/speedo is 10 to 15 mph slower than GPS . Or should I just toss the stock unit for a new GPS Speedo .

I have the same problem Good question

mine reads slow also, i tried drilling[enlarging] the brass adapter that screws to the pitot tube but all it did was make my needle bounce, i have another adapter to put back into it, looks like its time to go into the gauge itself

Send it to Gaffirg.. If its that far off something is seriously wrong... My Gaffrig (Livorsi) is within 1 - 2 MPH at its worst.. I am using the transom mount P/U though... And, I am "assuming" You have already checked the lines to the back and the water pickup..??

A water pressure driven speedo will rarely be close. Not trying to upsell you, but you would be better served with a GPS unit if you can swing it.

Is it Livorsi or Gaffrig? Send me a picture I can tell you.

If I can get you pricing let me know.

Kirk