All Garmins store the Date and Time for each track point and these are transferred to the PC when the track is downloaded from the GPS. This allows the speed to be calculated.
Lowrance / Eagle
Lowrance and Eagles do not store the Date and Time with each track point.
However they do allow the track point to be collected at set time intervals. If the track is collected at a set time interval then the "Set Track Date and Time" option in OziExplorer can be used to add the Date and Time to each track point.
Magellans do not store the Date and Time for each track point.
Adding the Date and Time to Track Points
This should only be done if the track points have been collected at a set time interval and the interval must be known.
If the option is used on tracks collected in "Auto" or "set distance" modes the speeds calculated will not be correct.
It is assumed that the track points were collected precisely at the intervals set, inconsistent speed values may appear if the GPS collects the points "on average" at the set interval instead of precisely at the set interval.