Compare two tire sizes — overall diameter, sidewall height, and the resulting speedometer difference.
225 is the tire's width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), and 17 is the rim diameter in inches. The calculator uses exactly these three values to compute overall tire diameter.
A positive error means your actual road speed is higher than what the speedometer displays — this happens when the new tire has a larger overall diameter, since each wheel rotation now covers more ground than the speedometer (calibrated for the original tire) assumes.
A larger-diameter tire has a larger circumference, so it needs fewer revolutions to cover a mile; a smaller tire needs more. Comparing revs/mile between the original and new tire is another way of seeing the same size difference that drives the speedometer error and odometer error.