I mostly don't use the HR strap with my 500 because it gets irritating sometimes. But I've noticed that the 500 overestimates the calories burned, IMO, on a ride without the use of the strap. I was on the North Branch Trail this past Saturday, finished a 60 mile ride, and burned something like 3300 calories in 3 hours. No real climbing. My avg speed was 18mph. Even at 175, I think that's overestimating.

I've just learned that it's a loose gauge of calories burned.