What tire width for 9 wide?
50-20 is typically fitted on a 9.0″ wide wheel, therefore the tire width is 12.5″ with the wheel much narrower than the tread width. For sports cars, the general rule of thumb is to match the wheel width to the tread width in inches.
Will a 285 tire fit on a 9 rim?
As noted in the first post above, most 285/30-18 tires are approved only for mounting on rims 9.5-10.5 inches wide. I would not recommend mounting them on rims narrower than specified.
Do wider rims need wider tires?
As a general rule of thumb, it’s safe to fit a tire up to 20 millimeters wider than stock on the original rim. The actual width of the tire will vary depending on the width of the rim: The tire will expand 5 millimeters for every half inch (12.5 millimeters) increase in rim width.