What do you think burns more body fat?

This doesn't make much physical sense to me. If work is force x distance, the distances are equal and the force is (pretty significantly) greater for the faster ride, how can the slower ride take more energy? How does Garmin assess calories burned? . I think you're forgetting about another major factor involved, time. Along with the force involved in going faster, it takes over a minute less time. To ride a mile at 20 mph would only take 3 min, but to ride the same mile at 10 mph would take 6 min. The Garmin uses HR, location/distance (by satellite along with data from the Internet), weight of the individual, as well as time to assess calories burned (cycling cadence is also a possible factor, possibly others). There are specific forums located at forums.garmin.com, which address this issue in far more detail.

I think you're forgetting about another major factor involved, time. Along with the force involved in going faster, it takes over a minute less time. To ride a mile at 20 mph would only take 3 min, but to ride the same mile at 10 mph would take 6 min. The Garmin uses HR, location/distance (by satellite along with data from the Internet), weight of the individual, as well as time to assess calories burned (cycling cadence is also a possible factor, possibly others). There are specific forums located at forums.garmin.com, which address this issue in far more detail. Another factor involved is efficiency. I think the heart rate correlates pretty well with the amount of work output, and the amount of time you can sustain that output is not linear. That's what you need to balance. The work in is not linear either. With swimming, you're overcoming drag forces to propel yourself. The energy you put into the water makes waves and dissipates: Big whorls have little whorls That feed on their velocity, And little whorls have lesser whorls And so on to viscosity. As you increase the energy you put in, more energy goes into the water. Similarly with cycling, you have both mechanical drag from driving the mechanism and wind drag increasing with speed. With running, I think it is just mechanics of the stride that causes efficiency to drop off with speed. To have the optimum "bang for your buck" in terms of time spent working out, you would probably want to do the exercise at which you are least efficient at as high an intensity level as you can stand. For me, that would be running at a very slow jog. But I detest running, so I'll stick with swimming for the time being.

I think the heart rate correlates pretty well with the amount of work output Maybe, but I don't believe it's completely true, I think HR is also affected by quite a few other factors (eg environmental, training fatigue) and also exhibits a lag. I think you're forgetting about another major factor involved, time. Along with the force involved in going faster, it takes over a minute less time. To ride a mile at 20 mph would only take 3 min, but to ride the same mile at 10 mph would take 6 min. If you are talking about the same course (same distance, same grade, all that), then no I don't think time enters into it. Extrapolate it out. Would it take even less energy to go 25 mph? 30 mph? Suddenly pro cyclists don't look so impressive... The main monkey wrench is changes in efficiency, as Fritz said. I don't think cycling technique efficiency changes greatly but the gearing (mechanical efficiency) obviously does. Maybe physiological efficiency changes some. But force goes up with the square of the velocity...it is really hard to imagine changes in efficiency would overcome that.