Understanding Tire Size: How to Read Sidewall Numbers

A tire's sidewall contains a standardized code (e.g., P225/65R17 91S) that defines its physical dimensions, construction, and performance limits. Decoding this information is essential for ensuring safety, maximizing fuel economy, and maintaining proper vehicle handling.

How to decode tire size: The "P225/65R17" Breakdown

Using P225/65R17 as an example, here is what each piece means:

• P: Is the Service Type. "P" stands for Passenger; "LT" stands for Light Truck.
• 225: Is the width of the tire, or the distance from sidewall to sidewall in millimeters.
• 65: Is the Aspect Ratio, or the distance from sidewall to sidewall in millimeters.
• R: Is the construction type. "R" stands for Radial (most common for modern cars).
• 17: Is the Wheel Diameter, or the size of the wheel (rim) in inches.

1. Physical Dimensions & Performance Impact

Tire Width (Section Width)

Measured in millimeters, width dictates the tire's "footprint."

• Wider Tires: Increase grip and stability but may decrease fuel efficiency and increase road noise.
• Narrower Tires: Often provide better fuel economy and superior traction in deep snow.

Aspect Ratio (Profile)

This number determines the thickness of the sidewall.

• Low Aspect Ratio (<50): Improves steering response and cornering (common on performance cars).
• High Aspect Ratio (>60): Provides more "cushion" for a smoother, more comfortable ride.

2. Construction Type

There are two tire construction types:

• Radial (R): The industry standard. Offers better tread wear, heat dissipation, and fuel economy.
• Bias Ply (D or B): Used for specific vintage cars, trailers, or heavy-duty agricultural machinery.

3. Service Description: Load & Speed

The numbers and letters following the size (e.g., 91S) define the tire’s capabilities.

Load Index (Weight Capacity)

Load index is the numerical code (e.g., 91) that corresponds to a weight chart. This number defines how much weight an individual tire can safely hold.

• Critical Safety: Never install a tire with a lower load index than what is listed on your vehicle’s door placard.
• Higher index is acceptable, especially for towing or hauling.

Speed Rating (Heat Management)

The letter (e.g., S, H, V, Y) indicates the maximum speed the tire can sustain.

• Common Ratings: T (118 mph), H (130 mph), V (149 mph).

Higher speed ratings often use softer rubber compounds for better grip, but they may wear out faster.

By understanding tire size numbers, you can make informed decisions when selecting tires that match your vehicle's specifications and driving needs. At SimpleTire, we are dedicated to helping you find the ideal tires for your vehicle, offering a diverse selection from top brands and expert guidance throughout the process. Shop for tires online with us and experience the convenience of finding the best deals from the comfort of your home.