How tire size is calculated
In 205/55 R16, 205 is section width in millimeters, 55 is sidewall height as a percentage of width, and 16 is wheel diameter in inches.
Sidewall = width × aspect ratio ÷ 100
Diameter = wheel × 25.4 + sidewall × 2
How a tire-size change affects the vehicle
A larger tire raises ground clearance by half the diameter difference and makes actual road speed higher than the speedometer reading. Width changes inner and outer space, but wheel offset, rim width and vehicle geometry still determine real fitment.
What tire diameter difference is acceptable?
The common ±3% guideline is useful for an initial comparison, not universal approval. Check the owner’s manual, load and speed ratings, approved wheel widths, fender space and suspension clearance.