Can anyone tell me why on a garmin 201 Displayed max speed is no where like down loaded max speed with realspeed or gpsar .Doneload with realspeed no spikes but no where near displayed speed( on 4 runs) 
Yep. Garmin save the data internally to 1/1000 of a nautical mile. Sounds good but that is ~2 metres. When you are doing 30knots you are travelling about 15m/s but your track position could be recorded at 14 or 16 from the previous because Garmin only record in ~2 metre steps. 2/15= 13% error. If you record every 2 seconds you can halve this error to about 6.5%.
The display speed is taken from the doppler calculation and is MUCH more accurate. The display is the true max speed. The software 2 second has a large error and because the software finds your best speed it shows the error values greater than your true speed.
The GT-11 measures 1/10,000 so is 10x more accurate on software trackpoint but also allows doppler recording (even more accurate) . The Garmin just shows doppler speed , it doesn't record it.
all the more reason to get a new gps.
