Tire Size Calculator

Metric tire codes like 205/55R16 encode width (205 mm), aspect ratio (sidewall height as a percent of width), and rim diameter (16 inches). Sidewall height = width × aspect ratio ÷ 100. Overall diameter = 2 × sidewall + rim diameter (converted to the same units).

Changing tire size affects speedometer accuracy, gearing, and clearance. A larger overall diameter makes the speedometer read slow; a smaller one reads fast. Stay within the manufacturer's recommended range when possible.

Example: 205/55R16 has a sidewall of 205 × 0.55 = 112.75 mm (4.44 in). Overall diameter ≈ 2 × 4.44 + 16 = 24.9 inches. Compare that to a replacement size before buying.

Aspect ratio is a percentage, not a measurement, which is why the same number means different things on different widths. A 55 series sidewall on a 205 mm tire is 112.75 mm tall, but on a 245 mm tire it is 134.75 mm. Two tires can share an aspect ratio and still differ by nearly an inch in overall diameter.

Keeping overall diameter within about 3% of stock is the usual guidance, because speedometer, odometer, ABS, and traction control all derive wheel speed from the factory rolling circumference. Beyond that, the speedometer error grows past most legal tolerances and stability-control interventions are timed against the wrong assumption.

Plus-sizing trades sidewall for rim: going from 205/55R16 to 225/45R17 keeps the diameter nearly identical while widening the contact patch. Steering response usually sharpens, but the shorter sidewall transmits more impact harshness, and wheels and tires both get heavier, which dulls acceleration and hurts fuel economy slightly.

The code carries more than dimensions. The letters and numbers after the size give load index and speed rating, and fitting a tire with a lower load index than the placard specifies can overload it at full passenger and cargo weight. Match or exceed both ratings on the driver's door jamb sticker rather than only the size.

Clearance is a physical check, not a calculation. A wider or taller tire can measure fine on paper and still contact the strut, fender liner, or suspension arm at full steering lock or full compression, so test both before committing — and remember the diameter change also alters effective gearing and therefore cruising RPM.

Plus-sizing (wider rim, lower profile) changes contact patch and ride comfort — verify clearance at full lock and over bumps before ordering, not just the rolling diameter math.