I can't find a listing on Dunlop's site for a 240/55 R18, only a 240/40 R18. I also searched other brands and came up blank as well. I wonder if that original tire spec in the owner's manual was incorrect. The parts manual calls for part number 41688-08 (updated to revision A at some point) and that part number translates to a 240/40 R18.
As for your question, if the stock tire really was a 240/55 and not a 240/40, there would be a big difference in overall diameter and circumference, which would alter the speedometer reading significantly. For example, a 240/55 tire would have an approximate overall diameter of 28.4", while a 240/40 would have an approximate OD of 25.6".
Jerry