Quote:
Originally Posted by 335diesel
Interesting about radii but surely tyres come with different profiles? It's not a set ratio. And go to a tyre dealer  the 19s look like truck tyres, the 17s don;t. You are also looking at fat rubber on 19s  say 265 so the profile has to be very slim to make it the same as a 17" 225/45.
I may be wrong on this but I can't see how "arch filling" 19s are the same diameter as 17s.

Try this link.
http://www.etyres.co.uk/tyresizecalculator.htm
Put in your tyres  225/45/17, and then 255/30/19. The difference in diameter is 2mm. My 17s are 205/50/17, which are actually bigger than the 19s by 1mm. Obviously the ratio can vary  but the idea is you make sure you get tyres/wheels that are the same radii for a given car otherwise the speedo wouldn't read right etc.!
Bear in mind the diameter of any given wheel will change by around 12.8mm from full tread to legal minimum, hence the difference between 17 and 19 diameter above is negligible.