maratoolHow to read tire sizes and compare 205/55R16 to another size
Type a tire code — get real dimensions and a two-size comparison with speedometer error.
That string on your sidewall is three numbers in three different units. The Tire Size Calculator decodes it into millimeters and inches, then compares any two sizes the way a tire shop would.
How it works
- Tire 1 — e.g. 205/55R16 (your current size).
- Tire 2 — optional candidate size to compare.
- Read the diff — diameter change and speedometer error.
The geometry
Per the ISO 4000-1 / ETRTO metric designation, sidewall height = width × aspect%, and overall diameter = rim×25.4 + 2×sidewall (all in mm). A 205/55R16 stands 631 mm (24.8″) tall with a 112.8 mm sidewall.
Why the ±3% rule exists
Speedometers, odometers, ABS, and gearing all assume the stock rolling circumference. Moving to a tire 3% larger under-reads speed by 3% and subtly raises effective gearing. The comparison panel shows exactly how far a candidate size drifts.
Sources & methodology
Methodology reference: ISO 4000-1 (passenger car tyre designation). Everything runs client-side in your browser; nothing is uploaded.
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