I think the S is soft. The M is medium. The H is hard. I have the M and S and the M feels harder than the S. Since HPI doesn't go by temp for rating you can't compare to other manuafacturers' tires. By normal rule of thumb you would use S in cold weather H in warm/hot weather and M in between.
-thanks! ..this sounds about right to me.
I've been running "M's" ...yet I think they are ballooning up too much on full throttle (as MikeR suggested) ...i'am actually seeing this happen. So researching new tire options-