QUOTE(etigge @ Mar 12 2015, 09:15 PM)
The Garmin is via satellite only. Usually 5 to 6 are tracked so, elevation and speed is quite accurate. Cateye is via the sensor but still the cheaper version has no minute settings only on general 16 inch, 20 inch, 22 inch etc settings. The more expensive ones you can set the settings to the exact diameter.
Ok, that explains the differences. I would say neither is really accurate for speed. I notice speed from GPS is less accurate at low speeds and not fast enough to detect speed changes.
Can your Garmin detect external speed sensors? Swap bikes and mount the Garmin on your wife's bike and see if you get the same reading difference.
Garmin units like the Edge 500, 810 and 1000 can manually set the exact wheel circumference.
QUOTE(azamunekurone @ Mar 13 2015, 10:11 AM)
IMHO better for me to secure it with cable tie to make it safer.compare to paid for lost magnets,that some price lah.sometimes for a long journey we never know what type of surface we will ride.unless the bike are on trainer than its ok for me

Neodym magnets are very strong magnets and flat and can sit flush in the lobang where the pedal is bolted. You can buy a few on the internet and very cheap.
Since they are very strong, you can even move the sensor slightly further nearer towards the wheel to detect speed.
QUOTE(desastar @ Mar 13 2015, 01:43 PM)
Aiyah, all these speed and cadence measurements are a waste of time.
If you are overtaken by someone, it means you are slow, so cycle faster and re-overtake that person.....
If your cadence is slowing despite shifting down, it means time to eat, then go home for a nap....

Well, I cannot speak for the rest but I'm a gadget geek and it's nice that I can take that part of it in this new found cycling hobby.

Different strokes for different folks.