Gps-speedsurfing
I was a bit bored whilst travelling recently and started thinking up little tricks to convert between knots, km/h and mph. All conversions use relatively simple mental artihmetic but may require a little practice before you can do them quickly:

http://www.mikeg.ws/gps/SpeedConversion.doc

After a bit of use, you start to remember common numbers so calculations become easier still.
STARBOARD