to convert between knots/mph/kph with a gps is very quick. and all stored data will also switch to the new units
on an etrex go to main menu/setup/units and then change to whatever units you want to display
on your computer if you are using garmin's map management software, mapsource, you can do the same to tracks saved years ago by going to edit/preferences/units and doing the same
guess i'm a geek but i try to take my gps along on every sail. the track files are tiny and download to the computer easily.
even tracks made years ago can now be opened in the latest version of mapsource through google earth. view/ view google earth
and if converted to .gpx files, (open in mapsource and resave as .gpx) the free version of GPSaction replay will display the track files as a color coded series of speed legs with an animated triangle racing around the course
the track i made when first seeing how high the hull would fly is quite instructional to watch animated as you see the boat speed along on a reach and then start slowing without changing angle much as the hull gets higher and higher and the rig dumps air. then when it has almost stopped you see the boat do the final little twist to windward to dump the hull back in the water
when my little lake community starts it's sailing races in the summer i will put my old etrex onto a friends laser and the new one on the nacra and we will get a better idea of how our boats sail the same course in the same winds
cheap gps and free software, best $100 to spend on your boat for safety, entertainment, racing and boat understanding