When making trucks of an unconventional size you sometimes run into wheel problems. For very large trucks, the wheels sink into the ground. For example, like this
<img src="http://mtm2.com/~forum/images/wheelssinking.jpg" width="640" height="480">
The same problem occurs for Bigfoot 5 replicas.
The problem is that in mtm the wheels ride at a constant height, so that any change in wheel diameter will result in the tire sinking below the surface, or riding above it.
For very small trucks, we can compensate for the tires riding too high by making tracks especially designed to accomodate them. This is where the -ST- or "small tires" tracks come from. They allow undersized wheels to look normal. This is achieved by making the texture depth deeper than on a normal track. The whole process handles slightly different than stock mtm tracks and trucks but it's quite manageable.
Unfortunately, nobody has figured out a way to make over-sized wheels look right.