Seeing as people are experimenting with sizes other than OE, I plugged all the typical sizes in to compare overall radius (affects the height of the bike) and compare the effects on gearing. Although its not recommended for the standard rim I included the 150/70-17 because some people have been using it.  The 4.5x18 rim is within recommendations for a 170/60-18 tyre (if you can find one).

Al