I find that my garmin is anywhere from 10-20% high in terms of calorie calculations vs. my Suunto HR which is more accurate..

I have done 20 mile rides / 70 minutes where it says 1800 kcal which is around 25 calories a minute.. Even at 90% + HR effort it should only be 16 cal per minute. Knowing my effort level it should be around 12 cal per minute..