I have the Garmin Edge 200. But it has never kept enough data, and I would have to keep using my Blackburn Delphi 6.0 to account for the data and HRM that my original Garmin didn't have.

Anyway, My parents' gave me the Garmin Edge 500 w/ HRM + Cadence Sensor, as a Christmas present. I have not been able to do a ride with the new Garmin because of the poor weather. So I was watching a comparison video of the Garmin Edge 200, Garmin Edge 500, and the Garmin Forerunner 705. During the video, the Garmin Edge 500(GPS) was doing the same thing as my Blackburn Delphi 6(battery) did. Where the speed would not be accurate.

So in terms of the Garmin Edge 500 in the video, would the constantly changing speed, be a sign of a bad battery, even with a full charge? My Blackburn Delphi 6 would sometimes go from 0-200mph and would jump all over the place. Which is what the Garmin Edge 500 was doing in the video.

Any GPS device will show your position moving around and an associated speed due to the inherent limitations/precision of GPS. These are usually on the order of 5-15 meters and a few miles per hour when stationary. A low battery will not cause or affect this as the unit will work correctly until the battery reaches a limit and then shut off before operation could become erratic. Garmin provides for a wheel sensor to help overcome this GPS limitation. It preferentially uses the wheel sensor for speed and distance which makes distance over tortuous paths more accurate, short-term speed more accurate, and the auto-pause function more reliable.

Don't worry, be happy. Set it up and use it on your bike. It will almost certainly work properly and well for you. If not, Garmin has very good customer service and honors their warranty.

Is the delphi wireless? Non-digital wireless can be subject to interference (producing wacky speeds). Otherwise, cyclce computers can miss detecting wheel rotations (producing low speeds). A low battery in a normal computer would generally report too low speeds.

GPS computers determine speed from position (none of them use doppler). GPS is more accurate for higher speeds. The Garmins have an option to measure speed from wheel rotation, which works when therewith is no GPS reception and it's more accurate too.

Regardless, I don't get odd speeds from my 800, used without a speed sensor.

The Blackburn Delphi 6 was not wireless.

Then, the odd high readings n the Delphi are not a normal issue.

If you have a cadence sensor, as long as the Edge is detecting it, it will draw your speed from that. "Jumping" speeds come from situations where the speed is being pulled from satellites and you haven't had the Edge on long enough for it to "lock" on to a satellite cluster. Generally, the more frequently you use it, the less time this takes, especially if you're in the same area a lot.

Set it up with a cadence sensor, make sure the Edge detects the sensor and you should be good.