Me and my GF did 22mi today. I wear a HRM connected to a Garmin 510 and she simply plugs in time and average speed into myfitnesspal. My calories from Garmin for the 1:45 ride was under 400 ... Myfitnesspal told my GF she burned over 1,000 for the same ride. Granted I'm in a lot better shape than her and my average heart rate for the ride was just 88, but I'm pretty sure the 1,000 cal estimate from myfitnesspal is way high for her ad well.

+1 to everyone who said if you want more accurate calorie burn data, get a HRM.