I have been using a Garmin Forerunner 305 for almost 3 years now. I never really paid much attention to the calorie count before. When I first bought it, I was doing the Weight Watchers Online plan, and it gives point calculations based on minutes of exercise.
Since acquiring my new Garmin Forerunner 405 and stopping the WW Online service, I have been watching the calorie count. I don’t really like the 405, so I started to use it for my evening walks and the 305 for my biking, running, etc. Well, one day, my 405 wasn’t charged, so I used the 305 for the walk. It turned into more of a walk/run, totalling five miles. At the end, the Garmin 305 told me that I had burned over 800 calories! Well, this was quite surprising as the 405 usually totalled no more than 460 calories. I wondered what was going on here.
Time for a (not so scientific) test. I made sure to set the profiles exactly the same in both units: Age, gender, weight, and then I put both of them on and started out the door. I have to say it was a bit weird having both units on at the same time, even though the 405 was on my other wrist. Watching one unit and then the next, I saw that the heart rate was exactly the same. Good so far. The pace was a bit different, but I had heard they did a better job of satellite tracking with the 405. At around the .25 mile point, I managed to stop the workout on the 405 (darned touch bezel!) before starting it again at the .75 mile point. In the end, the 305 registered 700 more calories burned than the 405!!
After some searching on Google, I found that Garmin uses the profile and distance, not heart rate to do its calorie calculations and that they are known to be essentially useless. I would have to say though that the 405 is probably closer to reality.