Looking to replace the stock speedo and would install this but looking for anyone that has some real world knowledge. Where does this get its GPS info from?

http://www.livorsi.com/catalog/merc_monitor.htm

You need a gps puck antenna to get speed or a complete gps system for data. if your boat has the 14 pin wiring harness 06 an up I think dont quote me on that you should have the plug for the harness under the dash.