A female friend who is 5' 3", approx. 140 lbs and 56 yrs. old and I ride together frequently. I am 5'9-10", 190 lbs and 65 yrs. old. On a typical 15 mph ride her HRM will indicate that she has burned about 60% of the calories that I have. I have referenced a variety of on-line charts, calculators, etc. They usually indicate that at 15 mph I'm burning 13-15 cals. per minute. These are all estimates of course but could the disparity be that much?