Obviously, my 993 is way out of warranty, so that isn't an issue, but is it worth worrying about the "N" spec. when you buy new tires or do most of you disregard it? Does it add anything other than Porsche's stamp of approval?

 

There are several excellent tires that are not 'N' rated (example - Yokohama Advan AD07), and many people here have their personal favorites (i.e. Michelin Pilot Sport 2s). IMO, buy good quality, have your alignment done, and monitor your tire pressures.

 

N tires are different than the non N tires of the same brand in their construction and load characteristics. For example here are the data for the PS 2 in the same size taken from the tirerack site:
235/40 ZR18 91Y SL N3 (Porsche OE) 220 AA A 1356 lbs. 51 psi 10/32" 24 lbs. 25.4"819
235/40 ZR18 95Y XL 220 AA A 1521 lbs. 50 psi 10/32" 22 lbs. 25.4" 819
or the Conti Sport Contact 2
205/50 ZR17 93W XL 280 AA A 1433 lbs. 51 psi 9/32" 19 lbs. 25.2" 827
205/50 ZR17 SL N2 (Porsche OE) 280 AA A 1279 lbs. 51 psi 9/32" 21 lbs. 25.2" 825

You see obvious differences in load ratings and weight of the tire between the N and the non N which signifies different construction and therefore characteristics.
Especially when it comes to 911's dont forget that there is no other vehicle out there with the weight balance of our cars so the N tires should at least in theory suit much better our cars.
Therefore if the tire you want have an N designation then go for the N. If you want another tire that is not Porsche approved, I would pay attention to the load rating on the rear tires to be high enough to avoid any potential issues...

 

So the N-rated tires are somehow significantly heavier than the XL rated versions from both manufacturers, yet have a lower load rating? Now THAT'S engineering!

 

Comparing apples to apples (at least in sizing):

265/35 ZR18 Yokohama Advan AD07 (Non "N") 1433 lbs. max load, 28 lbs

265/35 ZR18 Michelin PS2 ("XL" load, still not technically "N" rated) 1609 lbs max load, 25 lbs

So, you can see that not having an "N" rating really doesn't matter. Look at the load numbers, tire weights, and quality.

 

Those aren't apples. The Yoko has much heavier construction with stiffer sidewalls. (which makes no sense given the load rating, but the Yoko is probably downrated because of it's intended use.)

 

That's why they're on my car

 

I meant 'apples' re. not having an N-rating, but still being damn good tires, and certainly appropriate for the 911 re. load rating.

 

The N spec tires generally have an identical exterior aspect to other tires of the same size by that manufacturer; however, there are frequently numerous internal construction tuning 'tweaks' made to accommodate Porsches ( in this case) handling requirements.
There are many such tuning adjustments, but a few examples are the torsion/tension balance in the stranded cables of the 'crossed' under tread belts, the extent of cut overlap of these belts, deviations of the laid angles of these cables, pretension of the radial plys, on & on & on .... there are many dozens of such construction tweaks, and many more in the formulation and profiling of several of the extruded elastomer components of the pre cured tire.

Whether or not the subtle handling differences are still relevant to a 10 year old suspension design ( for tire technology always advances too), or to a suspension that now wears Bilsteins or WY&R coil overs, and is lowered is open to debate; however, at the time of design and construction - these adjustments can be felt in the car: many sets of test tires are made for on road testing to negotiate the final 'N' spec. to the satisfaction of the client.

 

With respect the AD07, there is a special "Lotus" version of the 225/45-17. It's the rear tire for an Elise. Since that's my front tire size, I wonder what the difference is/if it would be a good choice?

 

In my example both tires are front sizes. Dont forget that 911's have very little weight up front. You need traction for turn in, so the way the tire is constructed and its characteristics are vital in this case, not the absolute load rating. Like I said the differences are a clear indication of the different construction. As far as I know Porsche has specific requirements for handling etc to brand a tire N spec. This is always good no matter how you look at it. Are there any tires that could potentially be better than an N spec tire? Probably yes, but the fact of the matter is that if you decide to go with a brand and tire that has N spec line, then better get the N spec...

 

All I can tell you is that I've had quite a few different sets of tires and the AD07 is impressive for both grip and predictability. As you mention, the sidewalls are reinforced which makes turn in sweet , and despite the non-N spec, the load ratings are more than adequate for the rear end. As a nice bonus, even the wear is decent for a relatively soft compound. Pricing seems comparable to PS2s.