I pulled my specs from the chart provided (too lazy to do a roll-out), and my Garmin Fortrex 101 agrees with the Cateye on my bike to within .2 mi over a 60mi trip...

Most of the error is probably on the GPS too, since it may 'cut' corners or induce random placement errors (one severely erroneous point can add hundreds of feet to your overall distance). A properly mounted, WIRED cyclecomputer rarely incurs any random error... Wireless is a different story.

...and let's not even talk about coverage, GDOP and other sources of errors with GPS...

The short answer is: it all depends