Without having a power sensor attached to the garmin it's difficult to tell. It's so inaccurate using just the device alone. For example for my commute today my garmin said I'd burned 543Cal while the same ride at the same speed last week on the same device stated 315. That's 30 minutes of cycling on my heavy CX bike at an average of 17mph.1000 kCal/hour would mean riding at about 270W. Most people couldn't ride at 270W for 4.5 hours.
I do the same route on my carbon bike last week as well and that only took 26 and a half minutes averaging 20mph and that apparently used 364 calories.
I don't understand how a garmin can work any of this out without a HR, cadence or power sensor attached.