Car speedos are normally out and are normally optimistic.(read high) Manufacturers will usually err on the side of caution with this sort of thing. Better to have it reading slightly too fast than read too slow. If it was proven that their speedos all read slow, they might find thtemselves being sued by people who got done for speeding, or worse,by accident victims.
As for changing wheel sizes, fitting larger wheels shouldn't make a huge difference as when you put larger wheels on, you should come down a size or two on the tyre profile. The rolling radius of tyres of different sizes is rarely exactly the same, but there are plenty of tyre size calculators on line that you can download and work out the closest tyre size on say, an 18" rim, to the original 16" or 17" rim.
Try these and see just how far out your aftermarket wheels have put you!
http://www.tyresave.co.uk/tyresize.html
http://www.miata.net/garage/tirecalc.html
Obviously tyre width make a difference too as the profile is expressed as a percentage of tyre width.