For 255/40's, I'd suggest 9.5" over 9" if handling has much priority. Even half an inch wider than 'measuring' will demonstrate better composure in the upper lateral-g ranges if you're paying attention (you should at least notice).
FWIW, 255/40's are OK'd for rims out to 10" wide, and max-width fitments are even sharper and more precise once you're driving the corners well above the average driver's pace.
'08 GT coupe, 5M, wheels, tires, pads, fluid, a few suspension mods . . . still almost stock height
'19 WRX, 6M (hers)
'01 Maxima 20AE, 5M (spare, winter driver)