I know that this tyre size issue keeps cropping up, not least because the tyre sizes for our cars are becoming less easy to source.
I came across this calculator that's a rather handy tool for looking at the effects of changing wheel or tyre sizes and thought some of you might find it interesting too.. It certainly makes life easier for figuring out the effects of different tyres on your speed/speedo. It is American so bear in mind the Americanisms but useful all the same.
http://www.rimsntires.com/specspro.jsp