I've got a cateye speed and cadence meter mounted to the bike - and runtastic/mapmyride and several others are way off compared to my meter (which is set correctly for 406 wheels). A difference of around 2-4mph depending on the app compared to my physical sensors. Just to be sure, I rode along side my son yesterday (he has a 700c bike) and we compared sensor speeds to make sure mine was reading properly.
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