I've noticed that the variance on MMR is terrible most of the time. On the prospect ride, I compared it to a couple of other people I was with (one had a cat eye and the other using the iPhone kit) and at the halfway point we only had a few tenths of a mile variance. On the Olentangy route that I do on lunch with the same guy using the iPhone, we've had over a mile variance and our average speed almost a 2mph difference comparing MMR to his iPhone.