A while ago we was talking about GPS speedometer's

[dshaff24]

http://www.egauges.com/vdo_mult3.asp?Type=GPS_Speedo&Series=Phantom_Mar&Units=E

I found some marine GPS speedo's!!

[87SunSportMikeyD]

$140 + requires a gps receiver.... Wonder if that is any signal or a specific part

[tomelenbaas]

The problem with those speedos is the range - 100+ mph for a boat that tops out in the low 40's is overkill. A couple of years ago I was looking for GPS speedos and found one that was made by Livorsi specifically for tournament ski boats. It's now available at Barts:

http://www.bartswatersports.com/catalog/Electronics__Stereos/Boat_Instruments/Satellite_Controlled_Tournament_Ski_&_Wakeboard_Speedometer/index.asp

I ended up going with a Perfect Pass Star Gazer system instead - it serves as both a GPS speedo and speed control in one unit. It was about double the cost of a GPS speedo kit, but well worth it. The Perfect Pass is the best upgrade I've made to my '88 Mariah.