How to Calculate Tire Outer Diameter, with a Chart of Common Sizes

Outer diameter = rim diameter + sidewall × 2

Seen from the side, a tire has the wheel (rim) at its center, with the rubber part of the tire (sidewall) above and below it. The outer diameter is the rim's diameter plus the height of the two sidewalls, top and bottom.

1. 1

   Convert the rim diameter to mm

   For 205/55R16, 16 inches × 25.4 = 406.4 mm.

2. 2

   Find the sidewall height

   Section width 205 mm × aspect ratio 55% = 112.75 mm.

3. 3

   Add the two sidewalls

   406.4 + 112.75 × 2 = 631.9 mm. This is the outer diameter.

4. 4

   Find the circumference and revolutions

   631.9 × 3.1416 ≈ 1,985 mm. Per km, it turns 1,000,000 ÷ 1,985 ≈ 504 times.

The figure of 25.4 for converting inches to mm comes from 1 inch = 2.54 cm. Converting between inches and centimeters is also covered in Converting Inches and cm.

Chart of common sizes

Here is the outer diameter of commonly used tire sizes, calculated by type of car. These are design values calculated from the marking, rounded to whole numbers for circumference and revolutions.

The chart shows that 195/65R15 (634.5 mm), 205/55R16 (631.9 mm), 225/45R17 (634.3 mm) and 225/40R18 (637.2 mm) have almost the same outer diameter despite different rim sizes. Lowering the aspect ratio as the rim gets bigger to keep the outer diameter the same is the basic idea of plus sizing (see How to Choose Plus Sizes).

What changes when the outer diameter changes

When a car's tire outer diameter changes, the following effects appear. A 1% difference is about 6 mm on a 635 mm tire, hardly visible to the eye, but it clearly shows up in the speedometer and ride height.

• Ride height: rises or falls by half the difference in outer diameter. An outer diameter 20 mm larger raises the ride height about 10 mm
• Speedometer: with a larger outer diameter, actual speed is higher at the same reading. At 3% larger, a speedometer reading of 100 km/h means about 103 km/h in reality
• Odometer and fuel economy display: distance traveled is recorded as shorter than it really is, so fuel economy also reads worse than it really is
• Acceleration and fuel economy: a larger outer diameter raises the speed per engine revolution, making starts a little heavier
• Clearance to the fender and suspension: the larger the outer diameter or width, the more likely they are to touch when steering or going over bumps

Even at the same 16 inches, going from an aspect ratio of 55 to 60 raises the outer diameter by over 3%. Be careful: size changes that keep the rim size tend to produce the bigger outer diameter differences. The relationship between the speedometer and Japan's vehicle inspection is explained in Speedometer Error and Inspection Standards.

Calculated versus actual outer diameter

The calculated outer diameter is the design value set by the standard. Real tires of the same size differ by a few millimeters by manufacturer and model, and get smaller still when compressed by air pressure and load or worn down. New passenger car tires have about 8 mm of tread, so a worn tire can be over 10 mm smaller in outer diameter than a new one.

• The outer diameter in manufacturers' catalogs is measured on a specified rim width and inflated, and can differ from the calculated value by a few millimeters
• While driving, the tire compresses under load, so the actual rolling radius (dynamic loaded radius) is a little under half the outer diameter
• Winter studless tires may have a slightly larger outer diameter than summer tires of the same size, due to their deeper tread
• When comparing, using calculated values for both the original and new sizes gives a correct picture of the difference

When choosing a size near the limit, don't decide on calculated values alone; also check the outer diameter in the tire manufacturer's catalog and the result of test-fitting on the actual car at a shop.

Comparing outer diameters with the tool

1. 1

   Enter the original size

   Enter your current tire's marking (e.g. 205/55R16) as it is, or choose from examples for kei cars, compacts, minivans and so on.

2. 2

   Enter the new size

   Enter a candidate size to see the outer diameter, sidewall height, circumference and revolutions per km side by side.

3. 3

   Check the difference

   Check the outer diameter difference (mm and %), the ride height change, the actual speed at 40 km/h on the speedometer and whether it stays within the guide for vehicle inspection.

4. 4

   Save and share the result

   Use "Copy comparison" to take notes, or send a URL with the settings to a shop or friend.

If you're unsure how to read the sidewall marking, checking How to Read Tire Sizes first helps prevent input mistakes.

FAQ

How do I calculate a tire's outer diameter?

Outer diameter = rim diameter (inches) × 25.4 + section width × aspect ratio ÷ 100 × 2. For 205/55R16, that's 406.4 + 112.75 × 2 = 631.9 mm.

What's the outer diameter of 195/65R15 in mm?

15 × 25.4 = 381 mm and 195 × 0.65 = 126.75 mm, so 381 + 126.75 × 2 = 634.5 mm. The circumference is about 1,993 mm, and it turns about 502 times per km.

What happens if I fit tires with a different outer diameter?

Ride height changes by half the difference, and the speedometer reading and actual speed diverge. With a larger outer diameter, actual speed is above the reading; with a smaller one, it's below. Odometer and fuel economy values also shift.

How much difference in outer diameter is OK?

Within ±1% is almost the same, and within ±3% is the common guide when choosing. Even within 3%, you need to check on the actual car that nothing touches the fender or suspension.

Why does the calculated outer diameter differ from the catalog?

The calculated value is the design value under the standard, and real tires differ by a few millimeters depending on the model, construction and rim width used for measuring. When comparing, it's reliable to compare the original and new sizes the same way (calculated value to calculated value).