I use a Garmin 305. I rode 20.5 miles today, about 1 hour 22 min, and I burned about 800 calories. When rode 73 miles 2 weeks ago, 4 hrs 23 min, it said I used almost 3000 calories. Even if you're in great shape, you must be burning 500 calories an hour or more....

I can ask my DH who is the Garmin wizard around here. Did you set it up for your gender, weight etc?