On the over counting of calorie burned, I seem to remember some research paper with findings that people actually put on weight because they felt the little exercise they did gave them license to pig out, and overstating the calorie burned by the various devices was one of the problem.

If you are using garmin without power meter, count 1/2 if it'sodd older model, e.g. 605 or 705. On the newer models, e.g. 500 or 800, it's better but still probably overstating by 25% or more.