I can certainly see a difference in the heart rate when I am just trying to power through mashing gears compared with TRYING to spin in an easier gear. Granted, since I don't yet clip in I am always "mashing" to some degree, but can tell a big difference in my HR.

Where I can really tell the HR difference these days is in spinning class. No matter how hard I hit it, I am rarely able to get it above 144 these days. Of course that is only a 45 minute class and the periods of effort are much shorter than they are on the bike...I just remember last fall when those same efforts in class sent it soaring to 168-170