True dat.
It took my about 30 seconds to find a set of like-new 17" stockers from a 3 on Craigslist, a mere $150. I suited them up with a set of Yokohama IC-somethingoranothers in p215-50r17 size. I thought these would be the closest in size, just based on the math (tire width in mm * aspect ratio*0.5 / 25.4 + rim diameter = overall diameter). Mathematically, a p215-45r18 = 25.618 inches diameter, and a 215-50r17 = 25.465 inches, a difference of less than 3/16 of an inch, or about 0.60%. Pretty much negligable. But as you may know, the size that the tire manufacturer stamps on the side doesn't always match up to a real world measurement.
Case in point: the stock rubber is mated to the odometer perfectly, as confirmed by GPS over hundreds of miles. No error. However, these snow tires (which should only show an error of 0.6%) are actually showing an error of 1.8%. Not that 1.8% is a big deal of course; it's actually comfortably close to stock. Just make sure you check your size before you mount them up; remember that your mileage will accrue at a different rate depending if your overall tire diameter is smaller (accrue mileage faster) or larger (accrue mileage slower).
Yeah, that might be a bit on the obsessive side, I know. I study way too much data every day at work, and the whole data analysis thing has crept into my personal life too... (stoned)