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Convert the above calculated tire height (aspect ratio) in millimeters to inches by multiplying the millimeters by .03937 (139.75 x .03937 = 5.5 inches). Then take the inches and multiply by two and add the rim size. Example: 5.5 x 2 + 15 (rim size in inches) = a 26 inch tall tire.
For example, a plus two fitment for a car with 16-inch original equipment wheels and tires would be 18-inch wheels and tires. As a general rule of thumb, Plus Two Sizing: Increases section width by 20 mm.