GT-31 stores Doppler speed measurements in to track. Doppler speed measurement is much more accurate than position based speed measurement. Garmin GPS devices are also capable of Doppler speed measurements, but they donít store them in to track. They store position and time and from this they calculate speed. Max speed reading shown on Garmin GPS is derived from Doppler speed measurement and is more accurate than max speed reading from Garmin GPS track.
Iím convinced Garmin hardware is capable of recording Doppler speed measurements in to track, only thing they would have to change is software, but they donít want to do this.
You can insert 2Gb or bigger memory card in Garmin eTrex Vista Hcx and many others, GT-31 is not only one with this capability.