[Tony_SS]

Originally Posted by SCOTI

That's also true for the tires.... The 20" wheel has weight further out vs. the 15" wheel. The 15" tire has more shoulder & weight further out vs. the 20" tire w/the shorter sidewall. You indicate you could feel it.... I sure couldn't in my truck.

You have to take into account the wheel and tire mounted as a whole. The rotational mass is greater around the edge of a 20" wheel/tire than a 15", thus is takes more energy to accelerate and decelerate the 20" wheel.
Here, read this:
http://www.pro-touring.com/forum/showthread.php?t=35886

Some highlights are:

Lighter is better, but the more important factor is Moment of Inertia, which changes based on the square (I = kmr squared) of the radius. If two wheels have the same weight, but one has a radius 1" (15" to 17") bigger, the MOI of the bigger wheel will be nearly 30% higher. MOI is a continuum (MOI can be calculated at any place along the radius, to get the total MOI of a rotating disk requires integration), but since most wheels concentrate the bulk of their weight along the outside (the hoops that hold the tire), MOI calculated at the outside edge is a decent approximation.

The best visual aid I have ever seen to explain it is this. Take a hammer, hold your hand as close as you can next to the head of the hammer and rotate your wrist. It will spin very easy. Then place your hand farther away from the head of the hammer and try to rotate it. The farther you move your have away, the harder it is to spin. No change in mass, but a huge change in the amount of power it takes to rotate it. This also applies to stopping it as well.

So, for example, if you have a wheel that is 30 lbs, but most of its weight is near the center, it will spin easier than a 20lb wheel that has most of its weight on the out edge. So its not just about total weight, but also where the majority of the mass is located.

Then multiply that by 4, and you see how easily a larger dia wheel can affect things.