i was amazed when i wore my HRM out on the bike for the first time. perviously i only wore it on the trainer inside over the winter. i would find it comfortable around 120-130 BPM range, but when i went outside the same percieved effort had my HR at about 140-160 bpm. i was surprised because i was only going like 17 or 18 mph on a flat. now, if i take a recovery day - it's small chain ring only and hardly getting above 15mph.