Sorry,Guess I always get caught up with tires having to be as wide as possible with my V having 800+ rwhp, with drag radials on the rear. Might be you'd prefer a little taller rubber with a better ride, But like has been said, Just have the tire store roll out a couple different size & see what you like, After all it is your car & for you to decide what looks best on it. Good Luckthe 235 is the wdith of the tire...the 50 is going to be the heighth of the tire...if you want to fill that gap without lowering it, i suggest the 235/50 over the 255/40...when you go to the tire shop to put these on, have them roll one of each size out there and go with what you like best
Isn't the speedometer calibrated differently for different models of the CTS, becuase of the different rolling diameters? Wouldn't oyu have to know what his OEM tire size was, in order to calculate the difference?With the 235/50/18 you will be about 3.2 mph faster than indicated with the 255/40/18 only about .4 mph fast not to bad.