The footnotes on the Lexus website point to around 15,000 miles or "substantially less" for tire wear with the 18 inch summer tires.
On the 350Z boards, there is some very good buzz about these English tires called Avon. Specifically the M500 for summer tire replacement (over here on the left side, we don't have a lot of use for winter tires). The buzz is that they are unbeatable for the price. I've not tried them, so I have no real empirical knowledge; I'm just passing this along.
Here's a link that my 350Z-owning friend sent me that describes the tires (with a big 'ol ton of customer reviews, which I like).
http://www.tirerack.com/tires/Sizes.jsp?ma...model=Tech+M500
A 225/40WR18 runs about $117 (uninstalled), and the 255/40WR18 runs about $165 (uninstalled). Note that these are 'W' rated tires, as opposed to the 'Y' rated that come with the car. This means that they aren't rated for as high a speed. Fortunately, the 'W' rating is still 168 MPH, which is faster than the IS350 can go (unless you get rid of the speed limiter, and "do a few things"). The 'Y' rated tires that come with the car are good up to 186 mph, which is just wishful thinking...
For fun, I threw two of each (thats the sizes I have on mine) into the shopping cart, and calculated shipping to Central California. Shipping was only $35 bucks from Tirerack.com. So that's about $600 +tax and installation, which looks to me to be a darn good deal. By comparison, the OEM Dunlop tires run $178 and $229 respectively (also from tirerack.com)
Avon also makes an all season tire - the M550 A/S that is also hugely popular. Reviews for them at Tirerack.com rank them number 1 in AS tires.
http://www.tirerack.com/tires/tires.jsp?ti...Tech+M550+A%2FS
I looked up pricing for them too (yeah, it's a slow morning at work so far). They're even cheaper at $115 and $134 respectively.
Be careful when "changing" the load rating of a tire. The load rating is the "w" or "y" or "z" or "h" letter (ie:245/45/zr17 91Y)
These LOAD ratings affect the way the car will handle and perform.
Manufacturers spend lots & lots of money on R&D and tire design. Think that those Dunlops or Bridgestones ended up on your IS by accident?
NO WAY .. it was carfully planned and executed by engineers.
The tires might look like the same, smell the same and feel the same but in NO WAY will they perform the same IF you change the load rating. So if you want to change brands, be very very sure to get the exact same tire .. always compare apples to apples. Otherwise if everything is the same EXCEPT for the load rating then you are comparing apples to pineapples.
The number, not the letter is the load index. The letter is the speed rating.
http://www.tirerack.com/tires/tiretech/tec...e.jsp?techid=35