Hi,
First off I know that cycling calories are not very accurate without a power meter. This is a separate problem. I was using my Forerunner 255 to track cycling for several months and the calories burned seemed semi-accurate. Then I got a used Edge 520 and started using it. Since using it the calories burned for same distance/effort is about half of what it used to be. The weird thing is that even if I try to use my 255 again the calories are about the same as the Edge.
When it was "accurate" it would say about 800 calories/hr (I'm about 175 lbs). Now it says about 400 calories/hr.
Another note. At the same time my VO2 max dropped in half for cycling. It was 60 and dropped to 30. This is weird also since I don't have a power meter on my bike so the only time it gives me VO2 max for cycling is when I use a trainer with Zwift. My running VO2 max is about 60 so this seems like it should be closer to that.
Garmin Support has only suggested it could be HR zones which I'm skeptical could make that big of a difference. However, I made sure they were exactly the same as they were when the calories was accurate and it was still off by a factor of 2.
In the end it really doesn't matter but it is annoying it doesn't work right and frustrating that there is no solution. Whether it is coincidence or not it all started when I started using the Edge but it affects activities done with the 255 also.
I've also checked my weight on in various places and it says it is correct. Maybe there's some place I don't see the weight is wrong but that seems to me the only way it could be off by such a large amount. However, I would expect if it had my weight at half what it was supposed to be then my VO2 max would be sky high since I believe it is /kg.
Thanks