The 215 refers to width - the "55" refers to the percentage of the width that equates to the size of the side-wall. ie 215/55R16
So 55% of 215 is less than 55% of 225. This affects to overall radius - therefore the rolling circumference - therefore the gearing.
So for example if you wanted a slimmer tyre without affecting gearing - you could fit a 200/60R16 in place of a 240/50R16. Sadly It probably wont fit the rim.
I run a 225/55ZR16 CAR TYRE rear - And a 140/80ZR17 (M/C rear) up front. Sometimes called a DARKSIDE front. Darkside apparently does not just mean car tyre.