Yeah,
1 knot = 1.150779 mph. I recomend this program:
http://joshmadison.net/software/convert/
it is free and converts any unit to any other unit.
If you are trying to compute speeds in SH3, you can measure the distance traveled using whatever method you want, and compute it using ( Distance(m) /Time(s))*1.944. This will give you speed in knots for what ever time interval you chose. The easiest time interval is 3minutes 15 seconds ( 195 seconds), at which time the distance traveled in meters is equal to the speed in knots. ( 800m = 8kts) etc.