At last i have now got my winter tyres fitted,-Nokian WR G2 205/55 R16 91H-(going skiing in Austria next week!).Having taken off my 18" alloys there is a difference of about 15mm in the diameter between the 2 tyres, so this now means that my speedo will be reading incorrect ,how can i work out what true speed i will be doing? thanks in advance.

Its not far out.

Speedos always over read anyway..They are designed to

Look at this:

http://www.miata.net/garage/tirecalc.html

Thats such a small percentage I wouldnt worry about it to much, and drive as you did before.

There is probably around 12 mm difference in diameter between a new and a worn tyre.