I have read several articles on paper and the internet about the weight of a bike and the practical noticeable differences (none according to the cycling magazines and webarticles) when riding on flat roads.

I realize that there is some debate on the moving and non moving parts of the bike, but in this aspect it's just the total weight of the bike that my q'n is for.

What puzzles me is the following:
Example 1: Imagine that you put a backpack on and put several waterbottles in it and start walking, cycling whatever.
When you put it of you back you will notice a difference when doing the same thing when you had it on your back.
Example 2: Imagine 2 identical cars and one is lighter than the other. The lightest car runs more fuel efficient than the heavier one.

According to the articles they say that a light bike and a bike (or the parts on the bike) that is heavier (about 1 to 3 kg's) there is no practical gain or anything that you will notice when riding either bikes.
I find that hard to believe and there for i would like to know if any of you gals can help me with this q'n.

My personal info is that i ride a specialized allez alu bike, i'm light build, i ride always allone and on flat roads (there are no hills where i live) with the wind (head/tail/side) as my companion. I don't do competition and my usual ride is anything between 50 and 80 miles a day.

To sum it: will i notice any diffirence on the road or my performance when i would theoretically would switch to a light(er) bike than i now have.

Can anybody throw in some light regarding this issue based on articles or on practical experience.

Lets hear it.