No, that would be about perfect. With that size tire though you might want to consider going 18x8" as that is the size wheel that most tires that size as measured on, and would allow you to mount a wider tire at some point if you wanted to. I wouldn't go narrower than 225 by choice unless they were snow tires, but going slightly wider (within reason) would be ok. Back to the point though, 18x7.5" would be fine.
225/40 size tires would throw your speedo off only a VERY negligible amount. Keep in mind that the speedo already reads a little fast by ~1mph or so, so a tire with a taller sidewall might actually make it read more accurately.