I want to stay with 16's...
 

but I want a wider tread. I am looking at a set of American Racing Wheels 16x7. I would like to stay with the stock tire height but maybe 225's on the width. Is this doable?

I believe what you want is right here in the general forum http://www.scionxdforum.com/forum/showthread.php?t=561

yep thats a good calculator