According to this
calculator, the 225/40x18 tires make 804 revolutions per mile as compared to 807 revolutions per mile for the 195/65x15 tires. In other words, not real difference. There are greater variations than that as tire tread wears. The 18 inchers appear to be a perfect "plus 3". As I said before, they look nice and if you decide to sell them should be worth something...or just keep them for those occasions when you want more grip for a day trip through the twisties.
The only reason I asked was that sometimes people go for some very low profile tires that wind up lowering the car and affect the speedometer as well as the cruise RPM. A friend recently had this happen by accident...it seems he dropped the car off to get some new tires and the shop accidentally put one size smaller tires on it. It corners like nobody's business, but tends to scrape on rough roads. His car gets good mileage anyway and so he did not notice it right away.
Cheers