They do come in different compounds, and generally the larger the tyre the harder the compound - just he way the importer has chosen to go.

They guys we run with that are running heavier cars (Commodores etc) are spewing because they have to run harder compounds. Even some of the late model cars (syliva's etc) are finding they have to run the harder compound due to the lack of availablity of the softer ones. One competitor even changed from 17" to 16" to get the softer compound.