Complete Guide to Understanding Tire Size Codes: What Every Number Means

Understanding tire size codes is one of the most essential skills for every vehicle owner. These seemingly complex alphanumeric sequences stamped on your tire sidewall contain a wealth of critical information about dimensions, performance capabilities, load ratings, and safety specifications. Whether you’re shopping for replacement tires, checking if a tire fits your vehicle, or simply trying to decode what’s written on your current tires, this comprehensive guide will break down every element of tire size markings and help you make informed decisions.

The Complete Anatomy of a Tire Size Code

Let’s examine a typical tire size that you might find on a passenger vehicle: P225/45R17 91V. At first glance, this combination of letters and numbers may seem confusing, but each segment serves a specific purpose and tells you something important about the tire’s design, capabilities, and intended use. By understanding these codes, you can ensure you’re selecting the right tires for your vehicle and driving conditions, maximizing both safety and performance.

Service Type Prefix: What Kind of Tire Is It?

The letter or letters at the very beginning of the tire size indicate the tire’s intended service type and the standards it was designed to meet. This prefix is crucial for ensuring you’re using the right tire category for your vehicle:

• P (Passenger): This is the most common prefix you’ll encounter. P-metric tires are designed specifically for passenger vehicles including cars, minivans, crossovers, and light-duty SUVs. These tires prioritize comfort, fuel efficiency, low noise levels, and well-rounded performance for everyday driving situations. They’re engineered to handle typical passenger vehicle weights and driving patterns.
• LT (Light Truck): These tires are built to handle the demanding requirements of pickup trucks, full-size SUVs, and commercial vans. Light truck tires feature reinforced sidewalls, stronger internal construction, and significantly higher load-carrying capacities. They’re designed to handle heavy loads, towing, and rougher conditions that would stress standard passenger tires.
• ST (Special Trailer): Trailer tires are engineered specifically for towed applications such as boat trailers, utility trailers, and RV trailers. Unlike vehicle tires, ST tires are designed only to carry vertical loads – they don’t need to handle steering or provide drive traction. Never mount ST tires on a vehicle’s steering or drive axles.
• T (Temporary): This designation appears on compact spare tires, also known as “donut” spares. These lightweight emergency tires have strict speed and distance limitations and should only be used to get you to a repair facility. Most temporary spares restrict you to 80 km/h (50 mph) and should be replaced with a full-size tire as soon as possible.
• No prefix (Euro-metric): European metric tires have no prefix letter and are designed to different standards than P-metric tires. While they often perform similarly, there can be differences in load capacity calculations. Euro-metric tires are becoming increasingly common as global tire markets converge.

Section Width: How Wide Is the Tire?

The three-digit number immediately following the service type prefix represents the tire’s section width measured in millimeters. In our example of P225/45R17, the 225 indicates the tire measures 225 millimeters from sidewall to sidewall when properly mounted on an approved rim width and inflated to the correct pressure. This measurement is taken at the tire’s widest point.

Section width significantly impacts tire performance in several ways. Wider tires generally provide a larger contact patch with the road, which can improve traction during acceleration, braking, and cornering. However, wider tires also tend to create more rolling resistance (reducing fuel economy), generate more road noise, and may be more susceptible to hydroplaning on wet surfaces. Common passenger car widths range from 155mm on small economy vehicles to 315mm or more on high-performance sports cars and muscle cars.

Aspect Ratio: The Sidewall Height Profile

The two-digit number after the forward slash represents the aspect ratio, which describes the tire’s sidewall height as a percentage of the section width. In our P225/45R17 example, the 45 means the sidewall height equals 45% of the 225mm section width, resulting in a sidewall height of approximately 101mm (225 × 0.45 = 101.25mm).

The aspect ratio dramatically affects how a tire performs and feels:

• Lower aspect ratios (35-45): These “low-profile” tires have shorter, stiffer sidewalls that provide sharper steering response, more precise handling, and a sportier appearance. However, they also deliver a firmer ride, offer less protection against wheel damage from potholes, and may be more susceptible to sidewall damage from curb strikes.
• Medium aspect ratios (50-60): This range represents a balanced compromise between handling and comfort, making it popular for sports sedans and crossovers that need both responsive handling and reasonable ride quality.
• Higher aspect ratios (65-80): Taller sidewalls provide more cushioning and absorption of road imperfections, resulting in a softer, more comfortable ride. These tires are typically found on trucks, vans, and vehicles designed primarily for highway cruising rather than spirited driving.

Construction Type: How the Tire Is Built

The letter following the aspect ratio indicates the tire’s internal construction method. This is almost always “R” on modern tires:

• R (Radial): Radial construction has dominated the tire industry since the 1970s and is now found on virtually all passenger vehicles. In radial tires, the internal plies run perpendicular (at 90 degrees) to the direction of travel, with additional belt layers running diagonally beneath the tread. This construction provides superior flexibility, better heat dissipation, improved fuel economy, longer tread life, and more consistent handling compared to older designs.
• D (Diagonal/Bias-ply): In bias-ply construction, the plies crisscross diagonally across the tire. While largely obsolete for passenger vehicles, bias-ply tires are still used in some trailer applications, vintage vehicle restorations, and certain off-road applications where their stiffer sidewalls provide specific benefits.
• B (Belted): This rare designation indicates a belted bias-ply tire that combines bias-ply construction with additional stabilizer belts beneath the tread. These are seldom seen on modern vehicles.

Wheel Diameter: Matching Tires to Wheels

The number following the construction letter indicates the wheel diameter in inches – the size of the wheel that the tire is designed to fit. In our P225/45R17 example, the 17 means this tire is designed for 17-inch diameter wheels. This measurement must match exactly – a tire designed for 17-inch wheels cannot be safely mounted on 16-inch or 18-inch wheels.

Wheel diameters on passenger vehicles typically range from 14 inches on older economy cars and compact vehicles to 22 inches or larger on modern trucks, SUVs, and performance vehicles. The trend toward larger wheel diameters has grown steadily over the past few decades, driven by both aesthetic preferences and the desire for improved handling characteristics that larger wheels and lower-profile tires can provide.

Load Index: Weight Carrying Capacity

The two or three-digit number after the wheel diameter represents the load index, a numerical code corresponding to the maximum weight each tire can safely support when properly inflated. In our P225/45R17 91V example, the load index of 91 indicates each tire can carry up to 615 kilograms (1,356 pounds).

Load index is a critical safety specification. Installing tires with an insufficient load rating for your vehicle creates serious risks including structural failure, heat buildup, and potential blowouts. Always ensure replacement tires meet or exceed the load index specified by your vehicle manufacturer. Here are some common load index values and their corresponding weight capacities:

Speed Rating: Maximum Safe Operating Speed

The letter following the load index indicates the speed rating – the maximum speed at which the tire can safely operate under specified conditions when properly inflated and not overloaded. In our P225/45R17 91V example, the V rating indicates the tire is approved for speeds up to 240 km/h (149 mph).

While few drivers ever approach these speeds legally, the speed rating also indicates the tire’s overall capability and quality of construction. Higher speed ratings generally indicate tires engineered to better handle heat buildup, provide more responsive handling, and maintain stability at higher velocities. Common speed ratings include:

• S: Up to 180 km/h (112 mph) – Often found on winter tires and economy vehicles
• T: Up to 190 km/h (118 mph) – Common on family sedans and minivans
• H: Up to 210 km/h (130 mph) – Popular on sport sedans and performance-oriented vehicles
• V: Up to 240 km/h (149 mph) – Standard on many sports cars and performance sedans
• W: Up to 270 km/h (168 mph) – Found on high-performance vehicles
• Y: Up to 300 km/h (186 mph) – Reserved for supercars and exotic vehicles
• (Y): Above 300 km/h (186 mph) – Ultra-high-performance applications

Always select tires with a speed rating that meets or exceeds your vehicle manufacturer’s specification. Never downgrade speed rating without understanding the implications for your vehicle’s handling characteristics and warranty.

Additional Sidewall Markings Worth Understanding

Beyond the main size code, tire sidewalls contain additional markings that provide valuable information:

DOT Code and Manufacturing Date

The DOT (Department of Transportation) code appears on all tires sold in the United States and indicates the tire meets federal safety standards. The most important part of this code is the last four digits, which indicate when the tire was manufactured. For example, “2523” means the tire was produced during the 25th week of 2023. This date code is crucial because rubber compounds degrade over time, even on unused tires. Most tire manufacturers recommend replacement after 6-10 years regardless of tread depth.

UTQG Ratings

The Uniform Tire Quality Grading system provides standardized ratings for treadwear, traction, and temperature resistance. These comparative ratings help consumers evaluate tire performance, though they’re assigned by manufacturers rather than independent testing. Learn more about interpreting these ratings in our comprehensive UTQG guide.

M+S and 3PMSF Winter Symbols

M+S (Mud and Snow) indicates the tire’s tread design meets basic requirements for mud and snow traction. However, the more rigorous Three-Peak Mountain Snowflake (3PMSF) symbol indicates the tire has passed standardized severe winter performance testing. For serious winter driving, always look for the 3PMSF symbol. Understanding these markings becomes especially important when choosing between tire categories – explore your options in our winter tire comparison guide.

Selecting the Right Tire Size for Your Vehicle

When choosing replacement tires, always begin with your vehicle manufacturer’s specifications. You can find these on the tire information placard (usually located on the driver’s door jamb), in your owner’s manual, or by consulting with a tire professional. While some size variations may be acceptable through “plus sizing” (larger diameter wheels with lower-profile tires) or “minus sizing” (smaller diameter wheels with taller-profile tires), significant changes can affect speedometer accuracy, ABS and traction control calibration, ride quality, fuel economy, and fender clearance.

Use our tire size calculator to explore equivalent sizes and understand how changes might affect your vehicle’s performance. When comparing options, our tire comparison tool helps you evaluate different models based on performance characteristics, user reviews, and pricing. Browse our complete tire catalog to find the perfect fit for your vehicle and driving needs.