[Pete_vB]

While there are rare cases when wider isn't faster, and even rarer cases when it's not grippier, they are in my opinion clearly the exception not the rule in our size and weight class. I generally prefer to drive on skinnier tires, and maintain the stock widths on my 1M for that reason (even going down to 18s much of the time to increase slip angle using OEM M3 wheels). However I do believe that wider would be quicker against the clock in nearly all cases, and have a fair bit of experience that proves that out. Consider that vipers run 335s with significantly less weight on them- no issues making them work.

It does depend on the type of tire, if the suspension geometry can hold the tire flat, etc, so it can get tricky. Put a 305 30 19 on a very light old 911, as I have, and you'll recognize that many variables in the cars suspension need to change in order to make the tire work. However that's going from something made for 175 70 15 bias plys to a tire that the designers wouldn't even recognize. In a 1M's tire, power and weight class it's much simpler.

I have a suspension analysis program that includes various tire models. They are generalized rather than specific, but they give a good point of reference. Going from a 265 to a 285 gives a slight but noticeable measurable increase in grip at out loadings, and importantly that grip comes at a quite a bit lower slip angle, so you get faster response. On a big track as robertm is referring to this will be compounded because you'll be less likely to overheat the compound, increasing grip further.

In the wet skinnier than we run is better. I'll argue that on the street usable feedback from the tire and slip angle is more important than width. For these reasons I like and personally recommend not going wider than stock on the street. But against the clock I agree that it's slower, even with the increased weight.

For reference, I've got cars that I compete with wearing 175 15s through 315 17s and 305 19s, with lots of stuff in between. The 175s bring some of the biggest smiles.