So I'll admit it. I tried it. I was so taken aback by the idea that my Garmin HRM could be inaccurate, and therefore cheating me out of burning those chocolate cake calories, I wore both my HRMs for my Friday dog walk.
Yes I wore the 2 wrist receivers and 2 chest straps. I verified, as well as I could, that the HRM were picking up their associated straps.
Through out the walk I checked both receivers. Both HRM were reading within 3 BPM of each other. At the end of the walk, roughly 40 minutes, 1.7 miles with an average speed of 2.6 mph, the HRM were within 10 calories of each other.
In checking out Garmin's site, they use speed, weight and time to determine calories burned. I bet my Nike Imara only uses weight and time, as there is no way for it to know my speed. As there is an added component to the Garmin calulation, it also adds a possibility for error. This is probably where the difference in BPM (and therefore calories) comes from. I can only speculate that the differences in the the calorie calculations increases as the BPM reading increases. I could also be completely off base. But this is the engineer in me and I didn't take Statistics for Quality Engineering for nothing.
What does it all mean? In the end, it's simple.
Less cake or more running.