Current tire diameter 17 inch (431.8 mm) + (225 mm * .50) = 431.8 + 112.5 = 544.3 mm Proposed 431.8 mm + (215 mm * .5) = 431.8 + 107.5 = 539.3 mm The 50 is the aspect ratio between the tire's tread width and the nominal sidewall height.
Minor correction: Formula is wheel diameter in mm (inches * 25.4) divided by two, plus sidewall height. This gives you rolling radius.
