Calories burnt is dependent on HR as well. Cant just work it out from speed alone. Sex, Age, Weight, HR and Av Speed are all entered into my HRM which uses this to give me calories. I burn about 100Kcal per mile, but I am over 3 stone overweight, therefore I burn more calories than a skinny bloke.
Thats with an avg HR of around 140-150bpm, Avg Speed about 13-14mph on my MTB