But as for grip, half-worn tires tend to grip better than brand new tires......at least that's what SCCA racer friend of mine said. That's why in the street divisions, the ones that restrict you to street tires, they use shaved tires.....essentially half-worn tires instead of new. Shaving the tires down decreases the squirm in the tread and the tires are somewhat grippier.
Like Tire Rack has on its site:
"any tread design breaks up the contact patch into smaller elements and additional deep tread depth (required to enhance wet traction) allows tread block squirm which will reduce dry performance. This means that tires typically provide their worst wet traction ... and their best dry performance just before they wear out."
"A shaved tire's tread profile will usually result in a slight increase in the width of the tire's contact patch putting a little more rubber on the road. The resulting shallower tread depths reduce the tire's slip angle, increases its responsiveness and help stabilize its cornering power by minimizing tread block squirm. Minimizing tread block squirm also reduces heat buildup and the risk of making the tire go "off" by overheating its tread compound. And in many cases, shaved tires used in competition actually have a longer useful life than tires that begin being run at full tread depth."
"Most tires begin with 10/32" of starting tread depth. Our experience is that shaving them to about 6/32" will provide a noticeable improvement and is a good starting point. However, it's also important to remember that once used, all tires will be much less effective during the next competition season. Therefore we recommend that tires be shaved to a depth that is sufficient to last for no more than one competition season. If it is very important to you to do well in the one event you run a year, a tire shaved to about 4/32" will further enhance your performance."
So, what I'm getting at is that a half-worn or less tire should provide better grip than a brand new tire, at least if the grip of the two different tires is fairly close to begin with. The notion that a brand new tire will grip, at least in dry, better than a slightly worn comparable tire is not quite correct........