I haven't tested that directly, mostly because it require that you carefully follow the same route multiple times. When you do tests to compare to the Jones counter, it doesn't matter what route you follow, since the Jones and the GPS are measuring the same route. Mike Sandford did do multiple rides of the same route with his GPS.
But you can certainly get an idea of the variability of GPS measurements from my tests, simply by checking the variability of the difference between the GPS and Jones measurements. In the small number of tests shown in this thread
https://measure.infopop.cc/eve/...9510622/m/8301003542the range of difference was from -0.33% to +0.34%
I have also gone on a couple rides wearing my Forerunner and with another GPS mounted on my bike. The difference between the two was similar to what I saw among different rides.
BTW, the chart in that thread shows that you are just as likely to get a short measurement from a GPS as a long measurement. It's not true that GPS "always measures long." It IS true that runners in races wearing GPS always run as long or longer than the SPR, by definition, so their GPS measurements are almost always long.