To see how off the garmin 305 is, I used it on a fairly fast beach ride down the SA trail.. Here is the crazy calorie data.. 2:24 ride for 48 miles gave me a calorie count of 3587 calories. Even at the max 16cal/minute threshold that should only be 2300 calories..
MY HR was 150avg - 178 max.. So take what the Garmin is telling you with serious skepticism.. I normally use a Polar or Suunto HR when I ride which gives more realistic numbers..