It's because of the aspect ratio of your tires. They're taller/smaller depending on the tire size. A 245/45R17 is a different size (height-wise) versus a 285/35R18, it's just like changing your rear gears out.
__________________ Sold - Black 2000 Mustang GT, gone but definitely not forgotten
- Silver 2007 Corvette Current - DHG 2001 Bullitt