Please understand that the tire industry has its own logic. While 19 and 18 inch tires may have the same NOMINAL size, in practice the 19 inch is smaller.Example: A 19 in. tire that has a "100" mm cross section should be 4 in. wide, but in practice most will measure about 3.5 to 3.75. However an 18" tire with a "100" mm designation will measure out to a full 4 in. Don't ask me why. This is the way it has been since before I got into this business in 1974. The stock 19 in. rear tire on current is a true 3.5 in. width and height for a nominal rolling diameter of 26 in. If the new tire is also a 3.5 in. tire it will be smaller. However these tires are not much more common than the 19 we now use. The common 18 in. rear tire is 4 in. high and wide, giving the same nominal rolling diameter as the 19 in. If they use a metric 100/100-18 we are still near the exact same diameter. Going to a 100/90-18 gives a ride height of 3.5 in., again 1 in. smaller diameter than the current 19. So yes, tire choice will have an impact on over all gearing. Four % is a really minor change. It may even enhance top speed by providing enough leverage to pull higher RPM in top gear. You can think this stuff to death. I would suggest seeing if you like it as it comes. If you don't it will be a relatively easy fix.
So more rubber on the road? Can't argue with that. A lower rear fender would be nice though.
So more rubber on the road? Can't argue with that.