225 is the width of the tire. 45 sidewall height (aspect ratio). Think you got the 18 part. I don't have an 'R', so hopefully an R owner can chime in if i'm wrong. 225 is too thin for the 'R', 45 is too tall for the 'R'. Not sure if a 225 would even mount on your rim properly. 45 probably would rub because of the increased height. <p>I think the 235/40 r18 is the factory tire. I think the only modification you could do would be a shorter sidewall (firmer ride-prob too firm-easier to damage rims-better handling-maybe). Going any wider on the tire would rub also.
As posted above, the 225 (in mm) is the width of the tire. IMO that is too small for the R and but they should be able to mount them on the factory pegasus rims, as I have heard of some people putting snow tires on that are 225's. For everyday driving however, you will probably lose some grip by going with a tire that is that thin. Also, too wide a tire will cause excessive tire rub on full-lock turns (i.e. u-turns or tight turns into a parking spot). <p>The 45 part is the size of the sidewall that you see on the outside. The factory 17in rims come with 235/45 sized P-zero's. The thicker the sidewall, generally the more comfortable the ride, as there is more sidewall to help absorb the shock from bumps.