Forget the math and the physics folks. The question was "when do you notice a difference?" The answer is when you write the check for the bike .

In all seriousness, the answer is immediately. I dropped from an 18 3/4lb bike to a 15 1/2lb bike utilizing the same wheelset and the same gear ratios. I'm no faster than I was on the heavier bike on the flats, but I definitiely notice the difference in the climbing. There's a noticeable difference in exertion as evident by my heart rate in comparison to my pace during various familiar climbs. I can maintain the same pace with a noticably lower heart rate than I have in the past. My fitness peaked at the end of the September, so I know it's not just the engine getting stronger.