Powertap G3 + Garmin Edge 800 = screwy power measurements

[ausrandoman]

I went for a training ride after work today. As I was pedalling along, I noticed that the 3 second and 30 second average power figures seemed too high. An hour into the ride, there was a steep down hill. As I free-wheeled down, the power reading went up from 30 Watts to 40 to 50, reaching 60 Watts (without pedalling!) at the 60 kph bend at the bottom of the hill. "Hmmm...", said I, "that looks like I might have to take off about 20 to 30 Watts from the average or normalised power."

Sure enough, when I got back, the power was obviously too high for the average speed. This is on a frequently used route. I know what average I should get.

So ... what's wrong with the hub and how do I fix it?

[nickobec]

Never had that issue with my G3 and Garmin 500 & 510. But recently purchased a P1, went out for a ride and my power was down by 80 to 100 watts, stopped, hit calibrate and back to normal.