It's supposed to be a nice day, so I brought my bike with me for an one-hour ride before I go home.
Last Saturday morning, it was 41 degrees, rainy and windy for my short ride. I brought the iphone with the "MapMyRide" feature activated. On this day, my bicycle computer reported 11.367 miles, while the "MapMyRide" said 12.01 miles. This appears to be a 5.6% difference in distance measured.
My confusion was increased when I was tripping through the settings of the "app" when I got home. First, I was happy to see (and activate) the "auto-pause" feature, which I suspect pauses the timer when I'm stopped at a traffic light. Second, I was super-excited that there was a "tire size" feature, and I quickly changed the tire size from 7005c. EUREKA! PROBLEM SOLVED!!
Later, I started to ponder things and realized that the iphone isn't connected to my wheels in any way, and that the speed and location information is taken from the GPS satellites, so there's probably no point in putting my tire size into this app. So I remain troubled at the disparity between my bike computer and the iphone "MapMyRide" app.
But on the good side, I was riding up a hill in the rain, and the asphalt was nice and new, and I was pedaling so hard and the slope was so large that I think I felt my rear tire slip under all my power. So I got that fun memory of riding in the bone-chilling rain.
I suppose my next step is to take my bike to a high school track and ride four laps with the computer and iphone on, and see what each says. I don't know if I can continue and stay in my right mond knowing that somewhere there is a truth between this 2-5% discrepancy between my trusty bike computer and new-fangled iphone app.