To check if your PowerTap is accurate, you can use a "Stomp Test", applying a known torque to the hub, and seeing if it's measured correctly. Unfortunately this isn't possible with alternative head units such as the Garmin 705, 500 or 310xt or other ANT+ units from specialized etc. On the wattage mailing list Brian Fitzpatrick pointed me at Quarqd a simple daemon that can read ANT+ sport data if you have an ANT+ USB stick such as come with the Garmin 405 or 310xt. Unfortunately it only runs on Mac's or Linux, but a virtual linux install had it working on my windows XP. The raw messages out of ANT+ aren't very useful however. So I knocked up a little Adobe Air application which reads the messages, and assists with the testing. You need to install quarqd and have it running, then you can start the AIR application, point it at the instance, set up your bike with the weight on the pedal, enter the bike details, and see how accurate your PowerTap is. Not ideal, as getting quarqd up and running is relatively painful in itself unless you're pretty geeky, but it's better than reading raw XML messages. Download Stomper application And the result? Our PowerTap's are pretty much accurate. As accurate as our weights anyway, maybe some accurately measured weights and some speedplay pedals to hang them off to check even more accurately, but I'm not that worried.