The only way to know how many calories you're burning is to weigh yourself naked before and after a trip at the same time of day (with all that entails). Each pound of fat is about 3500 calories so any loss of weight confidently attributable to fat (not longer hair, toenails, etc) can be attributed to a deficit in caloric intake.

[Total calories consumed on trip + (lb weight before - lb weight after)x3500 calories per lb] / total miles = calories per mile (for that trip).

Now who's gonna do that?

Measurements from HRM's are useless because you (usually on the better ones) have to enter a subjective "fitness level" that hopelessly skews results.

My feeling is that most tourists over estimate how many calories they burn. And by alot.