Tire Size & Comparison
Tire diameter, sidewall, circumference and revs per mile from the size marking; compare two sizes for diameter change, ride height and speedometer error.
Both tires’ geometry side by side, the percent change in overall diameter, the change in ride height, and the true speed when the speedometer shows a given figure.
Example: 205/55R16 is 631.9 mm tall; 225/45R17 is 634.3 mm — +0.38 %, so at an indicated 100 km/h the true speed is 100.4 km/h.
Read the sidewall,
get the geometry.
What the three numbers on a tire mean, how the diameter drives everything else, what changes when you compare two sizes, and what the marking does not tell you.
The size marking
A metric size like 205/55R16 gives the section width in millimetres, the aspect ratio (sidewall height as a percent of width) and the rim diameter in inches. Sidewall = width × aspect ÷ 100; overall diameter = rim × 25.4 + 2 × sidewall; circumference = π × diameter; revolutions per kilometre or mile follow from the circumference.
Comparing two sizes
The percent change in overall diameter is the number that matters: the speedometer and odometer, calibrated for the original size, read low by that fraction when the tire grows (true speed = indicated × d_new ÷ d_old), the effective final-drive gearing gets taller by the same fraction, and ride height changes by half the diameter change. Section width and sidewall changes are shown for clearance and feel. A commonly applied limit is ±3 %; the acceptable figure is the vehicle maker’s.
Nominal against real
The marking is nominal: two tires of the same size from different makers differ by several millimetres in overall diameter, and the manufacturer’s datasheet gives the actual diameter and revs per mile. A loaded tire rolls on a smaller effective radius than its free diameter, so real speedometer error is slightly smaller than the geometry suggests.
What the number does not settle
Load index, speed rating, the rim widths a tire may be mounted on, and whether the size clears the arch and suspension are the tire and vehicle makers’ figures — see Wheel Offset for the wheel side of clearance and Gear Ratio for the effect on gearing. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.