Got this from the regular Porsche newsletter ...
Within a given tire family, there doesn't seem to be much if any variation in tread pattern, but there are a lot of internal construction details that are also tunable variables. In my experience at one of the domestic OEMs, certainly any vehicle program that has any pretensions to any kind of performance will go through at least a couple of tire submissions, and you'd be surprised at some of the non-performance-focused, mainstream programs I've been involved with that have done so as well (sedans, basic hatchbacks, SUVs, etc.).It is very difficult to believe there actually are differences. You sure can't tell by looks except the the magical "N". Or by handling. They also make a Mercedes version, "M", a Ford version, a BMW version, and I can't imagine how many others. To suggest they each have their own magical compound/formula, their magical, all but identical, tread patterns and as a result, their own molds to form the tires just doesn't make sense. In fact it sounds like a business model from he_ll. Here's what I believe. The auto manufactures use the standard tires during their vehicle testing. If those tires satisfy them, they "certify" that fact and allow Michelin to designate tires with the magical letter addition to the side wall. Oh and charge a small premium for the effort it takes to add the letter to the sidewall. Years of reading and trying to understand the differences have done nothing to change my opinion.
That Billy Johnson / motoiq.com article is pretty accurate regarding the tire development process. In particular, the paragraph on p. 7 under the heading "How Tread Design, Compounding, and Construction Come Together" describes the tire development process for most of the vehicle programs I've been involved with. I can't speak to Porsche's N-rating process, but one thing they do that I'm not aware of other manufacturers doing is evaluate and approve new tires and tire models even after development on a particular platform has ceased. In this case, it wouldn't surprise me if they're evaluating off-the-shelf submissions from the tire company, but the tire company is probably selecting tires based on what they know has been approved in the past for that platform. Having said that, I have noticed that there aren't many newer tire models on Tire Rack that are N-rated for the Boxster, as @sectachrome points out. Maybe Porsche focuses on the 911 when it comes to approving new tires.I came across this article once upon a time that talks about variations in the Michelin Pilot Sport Cup 2 tires among those targeted for different vehicles. They show a picture here of what they claim are different tread designs, but to me it looks like one is just worn worse than the other.
They may look the same, smell the same, and be the exact same size; but not all Cup 2 tires are created equal, and for good reason. Michelin produces off-the-shelf…motoiq.com
Personally, I've never bought N-spec tires for my Boxster. For that matter, the current tires I'm running aren't even OEM size - my personal preference.