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​Rim Diameter- is the diameter of the wheel opening in the tire in inches".  That's right metric tire sizes use both metric and standard measurements.  This tire is a 215/60R16 so it has a 16" wheel opening.
Aspect Ratio- This is the height of the sidewall as a % of the section width.  The tire above has a 215mm section width.  The aspect ratio of this tire is 60%.  So the sidewall height is 215x0.60=129mm 

Section Width- This is the distance across the width of the tire in millimeters. This tire has a section with of 215mm.  Converted to inches that's 8.464"

​Metric= Section width/Aspect RatioR for Radial Rim Diameter

​                                     245mm           75%                                 16"

There are several ways to size tires.  The first we'll explain is the most common (Metric Tire Size).

Tire Size