Here is a link that may ne usefull to all of you people who are looking at chsnging wheels and tyres.
Car Bibles : The Wheel and Tyre Bible
For ttb99:
At about 2/3 down from the top of the link, look for
Why would I want to change my rims and tyres anyway?
It gaves you a comparaison between different sizes and rolling circumference.
This is the important bit you need to look for when changing sizes.
If you go down from 19" to 18" with your tire size:
you will be looking at 245/40/18.
Otherwise your speedo will be out