Originally Posted by adxm

Power data is the way to go. From my data - I find that it's ranged anywhere from 25 cal/mile (a super easy flat ride in a slow group) to around 50 (a century with ~14k ft climbing). Virtually all of my rides are 35-45 cal/mile. HRMs typically exaggerate calories burned significantly.

If you don't have a powermeter, you can get estimates of your power using the analytic cycling calculators - and just multiply your average watts by 3.6 to get (to a good approximation) calories/hr. So to burn 1000 calories/hr you have to avg 278 watts - for 500, 139 watts.