How accurate is the calorie expenditure information on the Garmin 305?

From my morning commute:

Garmin "Fantasy" Calories: 718

Powertap work in Calories: 419

Well the Garmin is the only computer I have. So I'd assume that it isen't very accurate? I put the info into my fitday account and for the time duration it said that I burned just over 1,000 calories. The problem is, I've hear that fitday and other online nutrition/fitness journals are not that accurate either.

I'm trying to zero in on my nutrition as I begin HIM training and I'd like to shed a few %'s of body fat and about 8 lbs. Having accurate calorie expenditure information is critical.

I don't know if you are trying to "zero in" on your calorie deficit for the day or what you need to take in on race day, but this is the best calculator I have found for race day: http://tri-talk.com/TriTalkNC/TriTalkNC.htm

It's a great starting point for your nutrition needs and then you can adjust from there.

I am not really a calorie counter but use this when i want estimates for whatever reason:

http://primusweb.com/fitnesspartner/jumpsite/calculat.htm

The best tool I've found for tracking weight loss and calorie deficit is Hackers Diet at http://www.fourmilab.ch/hackdiet/.

It won't ask you to track what you eat or how you exercise, it works on the theory of calories in - calories out = weight change.  It assumes that your weight change is because of fat loss and is converted at 3500 Calories per pound of fat.

As for your Ironman nutrition, I don't think your body can absorb 100% of the calories you burn.  You'll have to play with your nutrition during training to find how many Calories per hour your body can absorb (about 300/hour) and in what forms you prefer it, sports drink, gels, fig newmans, etc.

As for the accuracy of various devices, well Garmin is pretty far off as it relies solely on speed.  Climbing takes a lot of work, but is done at a low speed.  Descending is pretty easy, but done at a high speed.  Wind messes it up even more.  The powertap is pretty accurate as it measures work performed (torque X rotational velocity X time).  1Calorie = 4.2kJ but the body is about 25% efficient at converting energy into work so you get 1Calorie burnt for 1kJ of work performed on the bike.

As the poster above suggested, pick a formula and be consistent.  Make wise food choices during the day (larger breakfast, smaller dinner, 4 meals a day, each meal about 55% carbs) and watch the scale to see long term weight change.  Pay attention to fatigue and your ability to perform the next days workout to determine if you ate enough the previous day.

That 100 Cal/hr while sitting at your desk is your Basal Metabolic Rate (BMR).  That's where you get the basic recommendation that you should eat 2500 Cal/day to maintain your weight.  From here you add in the calories burned during exercise.  You don't want to include your BMR in that measurement.  The powertap measures work performed in kJ.  This can be converted to Calories burned as I explained above.

That data was from my morning commute.  It's 11.8 km with a 1000' climb over the last 4 km.  It takes me 40 mins on average.  

BTW, how does the 310XT and the 405XT record heart beat intervals when set to smart recording?

50.6

Sorry but garbage in garbage out. If I think I'm burning 500 more calories an hour while cycling then I really am and I aim for a calorie consumption that provides a daily defcit of 500 calories then I'm not going to lose a pound a week and I'm likely to maintain or gain weight. I know that I'm not going to be able to get an exact number of calories burned but you've got to be as accurate as possible. I can't just use some wacky arbitriary number and base my eating off that and expect to get the results that I want. 

Thanks for the advice everybody! Lots of good stuff to look at. Thank you!

Cool! Let me know what you find if you have time to look that up. I appreciate it!