I worked on plenty of cars in my life, I know the difference when one car is higher than another, this Buick is higher.
The official height of the both cars are different by 1", the 18" wheels add more to the height down low where I was
By just changing your wheels, to 18" and going narrower on your tires you could pick up at least 3". in appearance.
That throws off the speedometer, but there are ways to deal with that, electronically you can buy a tool, or just go on line to see the calculator.
With the calculator you can see how much your off at say 35, then 55, or your favorite speed then you can keep that in the back of your mind when your speeding down the interstate.