To burn the most calories, push a big gear at a high cadence. In other words, make the bike go as fast as you can for the length of the ride you are taking. If your legs are totally trashed at the end of the ride but you never ran out of breath, spin a higher cadence - meaning use a lower gear. If your legs don't get trashed, then use a bigger gear. When I was starting to train seriously, I rode away from home until I was pretty tired, then rode back. You can get a lot tireder and still ride than you think you can.