speedo

Hello all, I've seen this topic thrown around a bit but have never heard any definitive answer. I typically cruise my 2002 260 DA around 3500-3700 RPM. In calm conditions and little to no current, the built in speedo reads about 28-30mph while the GPS unit reads 20-24mph. To me the speed of ~28...

My speedo gauge always reads zero, the analog and digital gauge, Under all conditions under power or just cruising. What controls the output, any help would be appreciated thank you, the boat is a 2006 340 sundancer with 8.1 horizons