First off, don't really listen to the treadmill because it cannot tell you what extactly you are burning. They just take your weight, age, and speed and it calculates a rough estimate.
Think for a second... Who will burn more calories, someone running at @ 2 mph or @ 6 mph? You don't necessarly need to run more miles, you could run faster and save yourself sometime. Try keeping your heart rate at about 70-80% of your maximum heart rate.
If you want to try something different, you could do interval training, which most newer treadmills have. Interval training has been showen to help people with their weight. With treadmills there will be variation.... One type will speed up or the other type will incline more... but the idea is that you would jog for 1 min and then sprint for 30 sec... (each machine is different)