A 225/45 tire may rub because the width is the same, but the thickness or profile of the tire is larger.

And if you accelerate on the freeway a lot, then 18's wont be too bad because most of the slowness of acceleration is from the initial motion of inertia that the wheel has to take. If you're accelerating from a roll, it won't lag as much (although it still will, it's probably unnoticible).