Powertap and Garmin - speed reading

[Phil H]

Recently, when using my Powertap wheel, the speed (and distance) on my Garmin has been under-reading. Often half of the speed that the person next to me is getting. It's intermittent. I suspect that I shouldn't have updated my Garmin firmware as that often causes trouble. It seems not to be calibrating the wheel size to the rotation sensor properly.

I've Googled but there's nothing obvious out there. Anyone else had this?

Or is it that my old Edge 705 just needs replacing with something new and shiny.

[Alan C]

Just a thought.

My Garmin should be measuring the speed from the speed sensor, which is wirelessly connected to it, but I have my suspicion that its not detecting it (battery failed or something?). When that is the case it uses the GPS signal to measure speed. (as I know it did before I got the speed sensor).

When I used to ride around with only GPS speed, going along a road which was tree lined, especially when the branches cross the road, the indicated speed would drop off dramatically.

Perhaps something similar is happening to you?

[Phil H]

I don't think so. I ride sometimes with and sometimes without the Powertap wheel. Yes, when it's not on there I do get GPS dropouts (which manifest as a short period of 0mph followed by a spike of ... well I've seen 481mph coming out of a tunnel in the Alps). This is a consistent low reading for several miles.

I've replaced the battery in the Powertap and that all seems well. In fact the power data seems fine - about what I'd expect for hauling a fat bloke along (I know what 200W, 300W and 500W feel like).

Tempted to reformat the Garmin and go back a revision on the firmware. Or put the "nice" back wheel back on.

Interestingly, Tom borrowed that wheel for the President's 10 and his data for that looks fine.

[MattR]

[quote="Phil H"]Often half of the speed that the person next to me is getting.