Ply rating or PR is a rating that is applied to bias-ply tyres. A bias-ply tyre has its plies aligned in a criss-cross manner so that they overlap. Star rating or SR applies to all radial tyres. These are tyres where the plies are set at a 90-degree angle following the direction the tyre rolls. Both SR and PR are used to determine the strength of the tyre through the way it is constructed, and how much horsepower it can sustain. So, if the PR or SR number is higher the more pressure the carcass (inner chamber of the tyre) can handle, and is a good indication for when you are looking for tyres that can handle a heavier load.