There used to be a program that you entered your diff ratio, tyre size, etc. in and it told you how much your speedo is out.

I'm just wanting to check that 255/40R18's are correct tyre size for a 9" wide rim...

I don't think this will answer that question but here's the speedo calc link...
http://www.fordmods.com/forums/documents.php?doc=30
Paul

Yeah, that helps. Tells me that there's a 2.12% difference between the factory original rear wheels/tyres and the new ones. So when I'm on 100 km/h now, I'm actually on about 97.