QUOTE(Bruce Allert @ Oct 4 2004, 09:26 AM)
What is an "N" rated tire? Is that the speed rating?
it is not a speed rating.
it is an indication that the tire has been tested and approved by the factory. I've seen 'N1' and 'N2' tires, i think. might have been 'N0' and 'N1'... i do not think it's a grading standard - IOW - N1 is not necessarily any better or worse than an N0 - just different.
i'm not qualified to recommend a Boxster tire; i understand there were issues with 18" wheels on the early cars, and some people did it anyway, and sure enough they had some issues. i don't recall what issues, i don't follow Boxster issues much. there are BBS's like this one for the Boxster and a RennList Boxster list, which i'd suggest would be good resources for further research.
a quick search of the Tire Rack site specifying OEM tires showed N2, N3, and N4 tires.
if it's your car, you -can- do what you want, but Porsche has been kind of at the forefront of realising that the tire is one of the most significant of the suspension components and developed suspension settings with a tire's characteristics in mind - this includes stuff like weight, rotational inertia, spring rate, grip... for cars with active suspension augmentation (traction control, stability control, etc...) the designers make some assumptions on how the car will behave in response to certain inputs - assumptions that may or may not be valid for tires radically different than those tested.
for primitive cars like 914's and 911's before AWD and active suspensions, you can play tire and suspension engineer and test pilot without too much risk or weird consequence. once you start getting computers into the picture, it may make more sense to pay attention to the manufacturer's recommendations. Porsche doesn't like to leave too much performance on the table, and their testing budget is probably lots bigger than yours...