Tire Basics: Reading the Sidewall, Size Codes, Load Ratings, and Speed Ratings

Decoding the Size Code

Take a tire marked P225/60R16 91H and break it down:

• P — Service type. P = Passenger. LT = Light Truck. ST = Special Trailer. Metric (no letter) = European standard.
• 225 — Section width in millimeters. This is the width of the tire from sidewall to sidewall when mounted on the specified rim width and inflated to specification. Not the tread width — the full width including sidewalls.
• 60 — Aspect ratio. The sidewall height as a percentage of the section width. 60% of 225mm = 135mm sidewall height. A lower number means a shorter sidewall — 35 series tires are low-profile performance tires. A higher number means a taller sidewall — 80 series tires are found on trucks and older economy cars.
• R — Construction type. R = Radial. B = Bias-belted. D = Diagonal/Bias ply. Nearly all modern passenger and light truck tires are radial.
• 16 — Rim diameter in inches. The wheel this tire is designed to fit.
• 91 — Load index. A numerical code that corresponds to the maximum load the tire can carry at maximum pressure. 91 = 1356 lbs per tire. Load index tables are in every tire spec guide.
• H — Speed rating. A letter code indicating the maximum sustained speed the tire is designed for. H = 130 mph.

The overall diameter of the tire matters for speedometer calibration and transmission shift programming on modern vehicles with adaptive transmission control. Changing to a significantly different aspect ratio (even at the same rim diameter) changes the overall diameter and affects speedometer accuracy and transmission behavior. Calculate the overall diameter before recommending a size change.

Construction Type: Radial vs Bias Ply

On any modern street vehicle, you're working with radial tires. The "R" in the size code tells you that. But it's worth understanding what radial means, because it explains why the industry moved away from bias ply decades ago.

In a radial tire, the body ply cords run perpendicular to the direction of travel (radially, from bead to bead). Steel belts run circumferentially under the tread. This construction allows the tread and sidewall to flex somewhat independently — the tread area stays flat and wide on the road, while the sidewall can flex to absorb impacts. The result is better wear, better fuel economy, better heat dissipation, and better traction than bias ply.

Bias ply tires have cords that run diagonally across the tire at 30–40 degrees to the direction of travel, with alternating layers going in opposite directions. The sidewall and tread are more structurally integrated — stiffer, more robust for heavy loads and off-road abuse, but inferior for highway performance and wear. Still used on some trailers, farm equipment, and off-road applications where durability under abuse matters more than performance.

Load Rating and Speed Rating

The load index is a number from 0 to 279 corresponding to the maximum load capacity of the tire. This is the capacity at maximum inflation pressure — not at normal operating pressure. Never install tires with a lower load index than the OE specification. If the vehicle needed a load index 100, a load index 91 tire is undersized for the application.

Load range is a related concept used primarily for LT and trailer tires. Load range C, D, E, and F correspond to increasing maximum pressures and load capacities. A Load Range E LT tire can carry significantly more load than a Load Range C tire of the same size.

Speed ratings matter because they reflect the structural integrity of the tire at sustained high speeds, not just whether it can physically reach that speed. A higher speed rating means better heat management, better high-speed stability, and typically better handling. Downgrading speed ratings to save money compromises the tire's ability to handle sustained highway speeds — particularly in hot climates or long-distance driving.

Common speed ratings: S = 112 mph, T = 118 mph, H = 130 mph, V = 149 mph, W = 168 mph, Y = 186 mph. Z is a category that covers tires rated above 149 mph and is sometimes combined with a W or Y designation.

UTQG Ratings: Treadwear, Traction, Temperature

The Uniform Tire Quality Grading (UTQG) system is a government-mandated rating system molded into the sidewall. It gives you three numbers/letters:

Treadwear: A comparative number where 100 is baseline. A tire rated 500 theoretically lasts five times as long as a 100-rated tire under controlled test conditions. Real-world results vary based on inflation, alignment, driving style, and surface type. Use treadwear as a relative comparison tool, not an absolute mileage guarantee.

Traction: Wet stopping performance on a government-controlled test surface. AA, A, B, or C. AA is the highest. Most quality passenger tires are A or AA. A C-rated tire has poor wet traction.

Temperature: Heat resistance under sustained high-speed operation. A, B, or C. A is the highest. All passenger tires must meet at least C. A-rated tires dissipate heat more effectively and are appropriate for sustained highway driving.

Tire Selection for the Application

The OE tire for the vehicle was selected through extensive testing on that specific platform. The size, load index, speed rating, and even the specific tire model were chosen to match the vehicle's suspension tuning, braking system, and handling targets. Deviating significantly from OE specification changes how the vehicle behaves.

For a standard replacement: match the OE size, load index, and speed rating. Upgrade to a higher speed rating if desired — never downgrade. Select a tire in the same performance category (all-season, summer, winter) unless there's a reason to change.

When customers want to "plus size" (larger diameter wheels with lower profile tires): verify the overall diameter is within 2–3% of OE. Ensure the new size clears the brake calipers, fenders, and suspension components at full lock and full compression. Lower profile tires transmit more road impacts to the suspension — if the suspension is worn, the customer will feel it much more with a 35-series tire than a 60-series.