low calorie expenditure

Calories are calculated on the basis of the HR / RHR ratio (HR = heartrate, RHR = average resting heart rate). So it looks like your friend either has a lower RHR, or higher HR during the activity (or both in the same time). Especially if you do not wear the watch during sleep, your RHR may not be properly measured. When it is just acquired during the day, when you are rather active, it will be quite high, and hence the ratio HR/RHR and the resulting Calories will be quite lower.

I wear it while sleeping and have a rhr in the low 50s.  When I take a one-hour spin class, I burn more calories than a 2-hour outside ride averaging 18 mph.  Just not right.

Also, in the spin class, the bike has a power crank and using my watts the bike registers double the calories as my Forerunner 735. 

Based on my readings for calories on bike rides, hikes, rollerblading, the activities are a waste of my time when compared to just sitting around and burning 100 calories per hour.

In Strava, men my age riding the same distance on the same day on the same roads in a group burned 3 times the calories I did.

Besides the wrist I have used with handlebar mount and have even placed the watch on my thigh under bike shorts but the result is always the same.

Rather than the theory and algorythm explanation, I would like to know what's wrong, because it is, in fact, wrong.

Thank you.

Steve Van Horn

Rather than the theory and algorythm explanation, I would like to know what's wrong, because it is, in fact, wrong.

Not sure how you imagine, that we, here on the user forum, can tell you what's wrong, when you do not post any data that we could look at. I've wrote you about the principles how it works, so that you can check it yourself, but if you prefer that other users analyse your data, you have to post some.

Sorry, I didn't realize this was users. My question was intended for Garmin tech support.