So, im a little confused.... Ive been reading articles and some tell me that, to lose weight, I should be exercising at a Fat Burning Zone, which would be 60-70% of my Max Heart Rate...while others say for more calories burned and an overall better workout, I should be exercising in an Aerobic Zone which is 70-80% of MHR.

I used the Karvonen Formula and so I know both zones, but what should my target be?

SOOO confused!

Good question, I'd like to see if anyone has a good answer for this too.

I just bought an inexpensive Timex heart rate monitor, and the little information book it comes with states that at 60-70% you will "lose weight and burn fat," and at 70-80% you will "increase stamina and aerobic endurance."

I wonder if you were to do a test trial and workout for two or three weeks in the "Fat Burning Zone" and document any weight loss and then repeat the experiment in the "Aerobic Zone" and see what gives you the best results. Might be one of those situations where it all depends on your body and where it individually performs best?

The whole fat burning zone thing is bunk. so old school. workout at 80-85% of ur max.

Actually, workout wherever you feel comfortable because whatever you do, you burn calories and you build endurance. Working at 80 to 85% is REALLY hard to do for any length of time, at least it is for me. I can work at 70-80% for over an hour, but once I get over the 80%, I'm huffing and puffing so hard that I don't want to continue.

I agree with JMfan...the aerobic zone is best and the whole "fat burning zone" is just an old school way of thinking in the fitness world.
I also agree with part of what berryblondeboys said in that you should workout where youre comfortable. If your fitness level is not so great yet than you can do more harm than good by exercising at too high of a percentage. I also agree that something is better than nothing.
At the same time, you dont want to cheat yourself out of a great workout by not working at your hearts potential.
Also...80-85% is hard because its working you but thats what you need...ABOVE 85% into the 90% is where you can't sustain for very long (anaerobic zone). As you know from your calculations, using karvonen takes your heart into account so 85% of your karvonen range should be absolutely doable.
ps. I'm so glad people are using karvonen more and more, its so much more reliable and accurate than percent max.

The way I understand it, during long-duration, fat-burning zone exercise (low- to moderate-intensity), you burn mostly muscle glycogen at first, but after about 15 minutes, your body switches to fat as it's main fuel. On the other hand, if you exercise harder (aerobic zone), you will burn more glycogen and less fat.

Of course, if you exercise aerobically a lot or do weight training, it builds muscle which burns more calories in your sleep or when you are not exercising. So I guess there's no real clear answer.