How do I read a tire sidewall code like 205/55R16 91W?

Read it in order: 205 is section width in millimeters, 55 is the aspect ratio (sidewall height as a percent of width), R means radial, and 16 is the wheel diameter in inches. The trailing 91W is the load index (91 maps to a set maximum load) and speed rating (W is 168 mph). Matching or exceeding your factory load index and speed rating keeps a heavier or faster JDM car within spec.