I understand the confusion. Please do not be insulted if I have oversimplified the following explanation. I wish I could draw a picture, but try this, forget the tire is there at all. The car is suspended from a rope at that corner. Assume the road is a flat piece of glass. With the corner at a height level with the other three corners, the center of that hub is an exact, measurable, unchanging distance from the surface of the road below it right? That is the radius of our circle. Lets say for the sake of argument that it is exactly 12" from the center of the axle to the surface of the road. Now, cut a piece of carboard in a perfect circle with a radius of 12". It will be 24" wide--it's diameter. If you mounted this cardboard circle on the hub, it would just touch the surface of our glass road. Now lay that aside. Lower that corner of the car by 6". Now our measurable distance is 6" to the surface of the road--our radius. Construct another cardboard circle with this radius and mount it on the hub. It too will just touch our glass road. Set it aside. Now we have a 24" diameter cardboard disk and a 12" diameter cardboard disk. Now just for the hell of it raise the car back to it's original 12" from the road then raise it another 6" to be 18" from the surface of the road. Go through the same process of creating a carboard disk, this time 36" in diameter. Now we have 3 disks of three different diameters. Now put your wheel back on so you don't forget and drive off without it )
Now lets make a jig with a straight dowel serving as an axle. Place all three disks on this axle and pin them together. Notice we have quite a difference in circumfrense in our three disks. Mark off an arc of twelve inches on the largest and smallest circles. This represents the car moving one foot. With the size difference of those two disks it it easy to see that the larger circumfrence disk with turn the assembly less to cover that 12", then the smaller disk. Another example, if we rolled each of the two disks one mile seperately, the smaller disk would make many more revolutions than the large one. This equates to revolutions per mile. If you travel the mile at the same speed both times then the wheel with the greater revolutions per mile will also have a greater wheel speed. This is what the ABS system measures. Now this all equates to tires because as air is added, the car is lifted higher off the road(to a point) just like our example suspended by a rope. As more air is let out, the distance to the road decreases and our wheel begins to spin faster to cover the same distance as the other tire on that end thus creating a measurable difference in wheel speed between the two.
Damn that was long winded!

Hope it makes some sense! Been out sniffing carb cleaner and I may be just a bit addled. )

Doug
'99 Black GTP Coupe