Monitoring calories burned question

Snerk13

Hello all, I was wondering if any one could tell me which way of monitoring my calories is more accurate. I ride a mountain bike and I have a speedo/odometer that measures calories burned from my weight, riding time, age and speed. I figure this isn't too accurate since generally if I am going fast it is actually easy going and when I am going slow that is when I am burning the most calories (it is probably geared more for a road bike). If I use the canned exercise, it groups BMX, mountain biking and biking together, it also shows considerably more calories burned. Any idea which is better to use?
Thank you for any replies.