Linda, based on both our experiences with this, I'm going to do some research on the Garmin site and let you know what I find out.

We did the 15 mile TT course last night with some grueling hills. My time was 50 minutes with a 17.1 average. I did not wear my hrm and the Garmin said I burned 759 calories. Just for comparison I went to caloriesperhour.com and put the info in and it said I should've burned 690 which is pretty close. Had I worn the hrm, it probably would've said 350!