If you add 1/2" width (12.7mm) to the wheel plus add 10mm offset (ie, +30), the total amount the wheel would extend outwards would be 10mm + 6.35mm = 16.35mm. changing to a 245 tire (10mm wider) would add another 5mm protrusion (5mm on each side of tire). This would be equivalent to oem wheel/tire with a 21.35mm spacer.