235/50/18 sounds quite large.
I don't have an Axis but the stockies on the RS are 225/45/17 and my Nismo's came with 235/40/18's.
I would assume that the Nismo rims would be roughly the same as Axis stock 18's, with both being 8" wide.
The calculator says that my speedo should be reading 1.4% slow, but I have found that if I sit on the speed limit, I am holding everyone up.
As an aside, steer clear of Nankan and Kenda tyres.
I currently have Kenda's and although they are A M A Z I N G in the wet, when the temp reaches 35 degrees, they become very soft and squishy (not fun).
I would be looking at 235/45/18 or 235/40/18.
The 40's will have a harder ride though.