Planning on putting on (Yohohama AVS ES100) 235/40-18's and 265/35-18's on stock track rims. Will this fit? Or should I go with the stock sizes 225/45 + 245/45? How will it affect the handiling?

 

Those sizes are too small buddy. If you can fit it, go with 245/40 and 265/40 to get closer to stock. Those sizes are still not exact, but are probably close enough not to throw off VDC.

You can run the #s yourself here:

http://www.miata.net/garage/tirecalc.html

949 lunch next week!

 

I'm concerned about hadling characteristics. If I go 20mm wider (225 to 245 or 245 to 265) won't that affect how the tire "plows" in hard cornering?