I was just looking at my data for a long ride Sat. I'm at 167 pounds. 200 miles pretty flat, average speed 20mph, calories burned 6,500, equals 32.3 calories per mile. The only time I have gotten up to 40 calories per hour is on hard rides such as an 82 mile ride with 6K feet of climbing - 16.45mph average. On a 40 mile ride I did last week I hit 42.5 cals per mile - that was at an average speed of 17.8mph including a 30 minute interval at 22mph. Obviously body weight affects it as does intensity (hills) not just speed. Most of my recovery rides are at 30 cals per hour.