60 is not the width. It is the aspect ratio, AKA the height of the sidewall - the number is the percentage of the (section) width, which in this case is 225mm. A 225/60 will be "taller" than a 225/55 because it has more sidewall height. 60% of 225 = 135mm, 55% of 225 = 123.75mm.
In the end, in order to maintain factory clearance and not rub anything by going too big, and to avoid looking like you have four donuts by going too small, you will want to maintain the same overall height. The factory 225/60/16 tire has an overall height of ~26.6". If you are looking for wider tires for extra performance, 235/55/16 may be a reasonable size. Its overall height is 26.2", which results in plus ~1.8% speedometer error (as per this
nifty tool.
Some possible rim/tires combinations that typically work with Bonnevilles and still maintain reasonable accuracy (of course, with different rim specs, YMMV):
235/55/16
245/55/16
225/55/17
235/55/17
235/45/18
245/45/18
....now 225 how far can one veer from that? 215? 235? what changes if that number/size is messed with? odometer and gas mileage? handling? Thanks in advance!! 
