Tire Load Index vs Speed Rating: How to Read Both

Tire size gets all the attention, but the two characters after the size decide whether the tire is suitable for your car.

Where to Find It

The sidewall carries a size line such as 225/50R17 94H. After the construction letter and rim diameter comes the service description: a load index number and a speed rating letter. On some tires the service description appears only on one side, so check both sidewalls.

The Load Index

The load index is a code, not a weight in pounds. Each number corresponds to a maximum load per tire at the pressure specified by the tire maker for that application.

• Lower numbers mean lower capacity.
• A higher index than the original equipment means more capacity, but not automatically a better choice, since it can change ride and handling.
• A lower index than the vehicle requires is unsafe, because the tires may not support the loaded vehicle.

The vehicle's required load capacity depends on the car's weight, its gross vehicle weight rating and how it is used. Towing and hauling raise the requirement.

The Speed Rating

The speed rating letter indicates the maximum speed at which the tire can carry its rated load, under controlled test conditions. Common letters run from Q and S up through H, V, W and Y.

Speed rating is also a performance indicator. Higher-rated tires generally use heat-resistant compounds and reinforced construction, which affects handling and ride. Fitting a lower speed rating than the manufacturer specifies reduces the tire's designed margin.

Why Matching Both Matters

A tire can have plenty of load capacity and too low a speed rating, or the reverse. Both are part of the vehicle's specification, and both appear on the tire placard inside the driver's door jamb and in the owner's manual.

Rules of thumb:

• Never go below the placard's load index or speed rating.
• Going up in speed rating is common when a preferred model is unavailable, but expect a firmer ride.
• Going up in load index can be useful for towing, but check that the tire still fits the rim width.

Mixing ratings across axles can affect handling, especially in an emergency maneuver.

Other Sidewall Details Worth Reading

• Maximum inflation pressure, which is a ceiling, not a recommended setting.
• M+S or three-peak mountain snowflake markings, which describe winter traction capability.
• DOT code, including the four-digit date of manufacture.
• UTQG ratings for treadwear, traction and temperature, which are useful for comparison within a brand's line.

Practical Buying Checklist

1. Read the placard for the required size, load index and speed rating.
2. Match all three, not just the size.
3. Confirm the DOT date is recent, especially on a bargain tire.
4. Set pressures to the vehicle's specification, not the tire's maximum.

For related reading, see what do motor oil numbers mean for another example of decoding a specification.