kdelong said:
Just a heads up to all of you GPS users. The GPS Signals that you receive on your units are a little off on purpose. It would not be a good thing for just anybody to be able to get super accurate coordinates and rates of speed, etc. Therefore, the satellites transmit a slightly skewed signal. U.S. Military receivers are made to compensate for this but civilian receivers are not. Don't put too much stock into the absolute accuracy of your GPS over your on the bike sensor computers.
All that has been removed since US Military's GPS is now potentially trying to compete with Euro's Gallileo system. They can still wipe out service to any region through jammers and what have you, but they don't degrade the accuracy of those signals or civilian aircrafts and other services would have major problems.
Going back to the OP's observation. Please bear in mind a number of issues,
1) Flight deck measures speed through tyre rotation and tyre's external circumference. So if the initial setting is not accurate or the tyre pressure is off from the initial calibration or there's wear in the tyre, then it will over or under estimate.
2) GPS measures straight line speed based on location plots taken at regular intervals. So it does not pick up any wavy movements of the front wheel or minor pot hole swerves.
So overall, I would trust the GPS data more as long as the signal is good and the sampling frequency is sufficiently great.
As for the altimeter accuracy. I am not sure what's the built-in altimeter design is in the 205. For the case of 305, it's a barometric meter with continuous additional calibration using GPS data. This method is the most accurate for regular use as it has the higher resolution of a barometric meter with GPS calibration to correct for atmospheric pressure changes during long rides (talking about high/low pressure cell movements). If 205 only has a GPS based altimeter, then yes, it won't have the higher resolution of a barometric based sensor, but would be good enough for significant hills more a few metres in height.