Does your HRM have a chest strap ? If not, then I would be skeptical of the calories burned. But a nice HRM with chest strap that takes weight, age, min or max HR into account would be quite accurate. And I think 1760 cals for a 56 mile ride sounds totally reasonable. On my 1.5-2 hour rides I regularly burn in the neighborhood of 900-1200 cals so I think your total seems normal.