I have my new magnet-less speed sensor on the rear wheel and have set the Edge 510 to automatic wheel circumference. After a few rides it has settled on 2065mm for my 700c/23mm wheels. According to http://www.bikecalc.com/wheel_size_math and other calculators the circumference should be more like 2100mm. Of course this depends on the tire and tire pressure, but a 35mm difference seems a lot.
What is your experience with the automatic wheel size calculation? One should think it gets very accurate after a while.. I just have doubts when the difference is so large relative to the expected value.