Car Tyre Sizes Explained: Markings, Dimensions and Size Charts

Car tyre sizes are described by a standardised code printed on the sidewall, such as 205/55 R16 91V. The first number is the tyre width in millimetres, the second is the aspect ratio (tyre sidewall height as a percentage of the width), the letter indicates construction type (usually R for radial), the next number is the rim diameter in inches, and the final part is the load index and speed rating. Understanding these markings helps you compare tyres and choose the right ones for your vehicle and driving needs.

How to Read Car Tyre Size Markings

A typical tyre sidewall marking looks like this: 205/55 R16 91V. Each component provides specific information:

• 205 – Tyre width in millimetres (also called section width), measured from sidewall to sidewall.
• 55 – Aspect ratio: the sidewall height as a percentage of the tyre width. Here, the sidewall height is 55% of 205 mm = 112.75 mm.
• R – Construction type; R stands for radial, the most common type for modern cars.
• 16 – Rim diameter in inches. This is the diameter of the wheel the tyre fits.
• 91 – Load index: a code indicating the maximum load the tyre can carry when inflated to its specified pressure (91 = 615 kg).
• V – Speed rating: the maximum speed the tyre can sustain (V = up to 240 km/h).

Other markings often include extra information such as the tyre's manufacture date (DOT code), tread wear, traction, and temperature grades, or whether it is suitable for mud and snow (M+S).

Understanding Tyre Dimensions

Tyre dimensions are given in millimetres for width and percentage for aspect ratio, while rim diameter is in inches. This mixed system originates from the historical development of tyre standards.

Tyre Width

Wider tyres generally provide more grip because of a larger contact patch, especially in dry conditions. However, they may increase rolling resistance, reduce fuel efficiency, and be more prone to aquaplaning on wet roads. Typical car tyre widths range from 145 mm to 335 mm.

Aspect Ratio

A lower aspect ratio (e.g., 40 or 45) means a shorter sidewall, which usually improves steering response and cornering stability but gives a firmer ride. A higher aspect ratio (e.g., 65 or 70) offers more comfort and better protection for the wheel against potholes.

Rim Diameter

Larger rim diameters allow for larger brake discs and are often chosen for appearance, but they require tyres with lower aspect ratios to maintain the same overall diameter, which can compromise ride comfort.

How to Compare Tyre Sizes When Shopping

When comparing tyres for your car, it is essential to first identify the size recommended by the vehicle manufacturer. This is usually found in the owner's manual, on a sticker inside the driver's door jamb, or in the fuel filler cap.

Step-by-step comparison process

1. Note the exact size from your vehicle – e.g., 215/60 R16.
2. Check load index and speed rating – Ensure any replacement meets or exceeds the manufacturer's minimum specification (e.g., load index 95, speed rating H).
3. Compare overall diameter – If considering a different size (e.g., a plus-size fitment), calculate the overall diameter to ensure it stays within ±3% of the original to avoid affecting speedometer accuracy and clearance.
4. Consider your driving conditions – Prioritise wet grip or winter capability if needed; this may lead you to different tyre models within the same size.
5. Build a shortlist – List tyres that meet your size, load and speed requirements, then compare features such as fuel efficiency (rolling resistance), wet grip, and noise ratings from EU tyre labels.

Size comparison example

Suppose your car uses 195/65 R15 91H. You are considering an alternative size 205/60 R15 91H. The overall diameter changes:

• Original: (195 × 0.65 × 2) + (15 × 25.4) = 253.5 + 381 = 634.5 mm
• Alternative: (205 × 0.60 × 2) + (15 × 25.4) = 246 + 381 = 627 mm
• Difference: about −1.2% – within the acceptable ±3%, so speedometer reading would be slightly higher than actual speed by about 1.2%.

This is a minor change; many drivers would not notice it, but it is worth knowing.

Key Considerations for Building a Shortlist

• Missing information – If a listing does not specify load index or speed rating, it is not necessarily a poor tyre, but you cannot confirm it meets your vehicle's requirements. Treat that as missing data, not a defect.
• Trade-offs – A tyre with excellent dry grip may have higher wear or more noise. A fuel-efficient tyre may have slightly less ultimate grip. Weigh these against your priorities.
• Genuine drawbacks – For example, a tyre with a lower speed rating than required is a genuine drawback, as it may not be legal or safe for your vehicle.

Car Tyre Size Chart (Common Passenger Car Sizes)

The table below lists some common tyre sizes with their dimensions. Use it as a reference to understand the measurements.

Always consult your vehicle's manual or a tyre professional before changing size.