So to the OP, I would suggest you get a good idea of how many calories you burn in an as compared to your percieved exertion or heart rate. Use your PT on your road bike to get the numbers. Then when you are on your mtn bike (or track bike), go by PE or HR to get your calories per hour. It isn't going to be really accurate, but probably the best you are going to do. And it should be good enough - most measures of calories in food aren't that accurate anyway.