I just replaced my 2007 RX400h tires with the Bridgestone Alenzas 235/55/R18 V90. Goodyear only had 400 tread wear and these are 600 tread wear. B)
Try using thes tools for your tire trades, but pay attentention to your actual tire dimensions as they compare to the tool's preset values for each size. As long as you stay within 3-4% on the tire height, you should not have any "geometry" problems (just be aware of the speedometer effects).
http://www.miata.net/garage/tirecalc.html
http://www.1010tires.com/tiresizecalculator.asp
If you compare the OEM size to the 255/55-18 you can see how much change you can expect to fit into the wheel well.
My GY 255/55-18 ResponseEdge tires used to rub a bit on the flexible plastic shroud in the rear, but they don't rub on anything metal or fixed that would cause a problem.
