Interesting study TrojanHorse. To give the OP an idea how much MHR can vary the 220-Age puts me at 164. Actual value for me is 156 on a bike and 158 on an elliptical. I will say the perceived effort to get there on an elliptical is much less than on a bike. I think it has to do with being able to expend energy with my arms as well as my legs. It's like the more muscles you can recruit the less perceived effort it takes. Another note is the effect of heat and humidity. Later on in the season when it gets hot you may find that even though you aren't going all that hard your heart rate will rise. That's because you need more blood flowing for cooling especially if you have too much insulation like I did last July.