Tire Size Calculator

How to read a tire size

For 205/55R16, 205 means nominal section width in millimeters, 55 is the sidewall aspect ratio (sidewall height as a percentage of width), R identifies radial construction, and 16 is the wheel diameter in inches. A service description such as 91V refers to load index and speed rating; this calculator does not verify that rating for your vehicle.

How tire diameter is calculated

Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100. Overall nominal diameter (inches) = rim diameter (inches) + 2 × sidewall height (mm) ÷ 25.4. Circumference (inches) = π × diameter (inches). Nominal revolutions per mile = 63,360 ÷ circumference (inches).

Worked example: 205/55R16 and 225/45R17

A 205/55R16 has a 112.75 mm sidewall and a 24.88-inch theoretical diameter. A 225/45R17 has a 101.25 mm sidewall and a 24.97-inch diameter. The replacement is about 0.38% larger in nominal diameter. At an indicated 60 mph, the simple diameter-ratio model estimates 60.2 mph; actual speedometer behavior depends on calibration and real tire rolling dimensions.

What the diameter difference means

A larger-diameter tire travels farther per theoretical revolution, reduces theoretical revolutions per mile, and may change speedometer indication. A smaller-diameter tire moves in the opposite direction. The calculation does not establish whether either tire clears the vehicle or fits the wheel.

Nominal math is not actual tire fitment

Physical dimensions vary by tire model, wear, inflation, load and measuring-rim width. Actual rolling circumference differs from a no-load circle. Verify original-equipment requirements, wheel width, vehicle clearance, tire load index and speed rating, manufacturer specifications, and any applicable restrictions before buying or installing tires. A difference within 3% is not a fitment or safety approval.

Supported and unsupported tire sizes

This version handles common metric radial passenger-tire codes, including an optional P prefix and an informational service description. It does not handle off-road flotation dimensions such as 33×12.50R15, motorcycles, wheel offset, bolt patterns, or vehicle-specific fitment. The numeric acceptance range is a parser safeguard, not a tire availability database.

Privacy and local exports

Your tire sizes and results remain in this page's memory. No upload by this tool. You may copy a calculation summary, download a local CSV, or print and save a PDF using your browser. Nothing is automatically stored after leaving.

Common questions

Will the new size fit my vehicle?

Not necessarily. This page compares theoretical dimensions only. Fitment depends on the wheel, clearance, load and speed ratings, vehicle requirements and the specific tire model.

Why can actual speed differ from the result?

The displayed speed estimate uses the ratio of nominal unloaded diameters. Real rolling radius and instrument calibration can differ; compare the manufacturer's published specifications and verify speed independently.

For tire-marking definitions, see Michelin's tire markings guide. For dimensional context, see Tire Rack's tire-dimension calculation guide.