How to Read Tire Size
A 205/55R16 tire is not three arbitrary numbers. 205 is width in millimeters, 55 is aspect ratio, and 16 is rim diameter in inches. From those you can estimate sidewall, overall diameter and speedometer error.
What the three numbers mean
205 is section width in millimeters; 55 is sidewall height as a percentage of width; 16 is rim diameter in inches.
Sidewall = 205 times 55 percent divided by 25.4, about 4.4 inches. Overall diameter = rim plus two sidewalls, about 25 inches.
Speedometer error after a size change
A larger diameter makes actual speed higher than the speedometer; a smaller one does the opposite. The error is roughly the diameter change percentage.
Also check fender clearance, load index and speed rating, not just diameter.
FAQ
Is 55 a measurement in centimeters?
No. 55 is the aspect ratio: sidewall height is 55 percent of section width.
Can I change only the width?
Yes, but overall diameter changes, affecting the speedometer and clearance; keep diameter close to stock.