The web site kanbike includes a calorie calculator. When I first found this site about six years ago, I emailed the site's owner to ask, how would I adjust the calculator to account for the weight of an adult tricycle (maybe as much as 60 lbs.) having a single gear? (This was a long time before I had the trike converted to a three-speed).
He replied that the calculator is based on a bicycle weighing 25-30 lbs., and having multiple gears. He suggested multiplying what the calculator gave my by 1.75. That is how I figured that I burned about 4000 calories during my tricycle century. Given that I was fighting the wind most of the day, I may well have burned more than that.
Once I got the road bike, I asked kanbike about adjusting the calculator for a 17.5 bike. I thought, multiply the calculator's results by 0.66. He said, yes, that's about right.
I don't bother to check my calorie burn after every ride, although maybe I should. I could stand to lose as much as ten pounds as of this blog! The calculator can't take in all possible variables (hills? flats? headwind? tailwind?) but a rough estimate of my calorie burn is probably all I need.
So there's my take on this burning issue.