mine lists off every tyre option gtr's had with about 4 diff sizes, not sure what size they ment to read on there as there is a few 16inch sizes mentioned.
it also has 17's which only the vspec got i believe, so not sure why mine says it.
why u crazy fools feel a need to go 1 inch bigger, by my calculation its 5.555555555556 % bigger than a 18inch. not really noticable in my book.
go wider, getting 4 275's tyres are where its at