I’m pretty sure I know how a GPS works, but this has stumped me.
On my local cycle path, an old railway line, both my Garmins consistently under-read my speed, by as much as 20-30%. This is compared to my cateye computer. On the roads the computer & the GPS tie up. On the path, I had thought that because part of the route goes through a cutting, that it might be down to a poor satellite fix: However, the Edge shows a decent accuracy figure (in the order of tens of feet), the error consistently repeats & also the distance travelled pretty much agrees with the cateye (over about a mile). I don’t understand how the GPS can so consistently under read the speed but show distance the same. This only happens on this particular, relatively straight bit of path. How hum. 😕

is it near a military base? 😈

Are you also using a GSC10?

poor recovery from a multi-path reception of the signal?
(For example, GPS does not like running somewhere like New York where it gets a view of only a couple of satellites)

Are you also using a GSC10?

No just the GPS standalone

Are you sure its not the cateye over reading the speed?
Otherwise it is probably a loss of satellites and the Kalman filter trying to fill in until it gets enough satellites back to give a fix.