Answer: exceptionally accurate. Having used the Polar S1 Foot Pod with my Polar S625X and now the Polar S3 Stride Sensor with my Polar RS800sd on the same routes, I know that they are both equally accurate. However, that has presented a problem for me, because they both confirmed that I am now slow. Thinking that wasn’t possible and also needing to be smarter on my solo runs all over Albuquerque and in the Sandia Mountains, I decided to upgrade my Sprint phone to a GPS enabled Katana II and use Wireless Run Tracker to track my runs (and find me in case something happened to me on a run). Sadly, the GPS only confirmed what I knew in my heart – my S3 Stride Sensor was only one one hundredth of a mile off in a 45:00 run and I am slowing down. In repeated tests over the last three weeks, in all terrains and at all paces, the Polar S3 Stride Sensor has proven over and over that it is exceptionally accurate. With the speed, pace, cadence and running index information that the RS800sd with S3 Stride Sensor provides, it is the ultimate training tool for runners.