Heart Rate and Fat Burn

[Scotty.]

I ride a bike at the gym, and on the console it has a list that gives target heart rates by age for fat burn and for cardio. The fat burn is listed as 140 bpm (low intensity) and the cardio is listed as 160 bpm (high intensity) for a 20 year old.

Even at a relatively easy pace and resistance level, my heart rate gets in the 160+ range due to the ephedrine I'm taking. I would guess without it, it would still be 150+ because I'm kind of out of shape. I don't really fatigue at all, and don't get short of breath at the rate I'm going, but my heart sure gets pumping. Would it actually be more effective fat burning if I had my heart rate lower? To go much slower/easier would almost feel as if I'm doing nothing.