Read the service description as part of the tire specification

A tire marked 235/60R18 103H, for example, includes more than width, aspect ratio, construction, and wheel diameter. The final number and letter are commonly called the service description: the number is the load index and the letter is the speed symbol.

The load index is a code that corresponds to a maximum load value for the tire under its rating conditions. It is not the same thing as the vehicle payload rating. The vehicle can still be overloaded even when each tire has a high load index because axle ratings, GVWR, suspension, wheels, and other components also set limits.

The speed rating describes the tire’s tested speed capability under specified load and inflation conditions. It does not mean that speed is legal or safe on public roads, and it does not override vehicle, road, weather, or load limits.

Do not substitute one tire number for another

These labels answer different questions, so a higher value in one column does not make a tire a universal upgrade.

Load index
Checking the tire’s load-carrying rating

Use the manufacturer’s required service description and vehicle loading limits. A higher load index does not increase the truck’s payload or axle rating.

Speed rating
Checking the tire’s speed capability under specified conditions

It is a tire capability rating, not a recommended cruising speed. Replacement guidance can differ by vehicle, so follow the owner’s manual or manufacturer fitment information.

UTQG grades
Comparing certain passenger-tire treadwear, traction, and temperature characteristics

UTQG is a separate U.S. grading system. It does not replace the load index, speed rating, tire size, or vehicle placard requirements.

Use this order when comparing replacement tires

  • Confirm the factory tire size on the driver-door label and in the owner’s manual
  • Check the original service description or manufacturer replacement requirement
  • Verify load index and speed rating rather than comparing only the tire size
  • Confirm the wheel diameter, width, offset, and tire type if wheels have been changed
  • Check whether the vehicle requires XL, reinforced, LT, run-flat, or another specific construction
  • Keep axle, GVWR, payload, and towing limits separate from tire load capability
  • For mixed-size, staggered, AWD, or performance applications, follow model-specific replacement guidance

Safety check

Do not lower a rating just because you never drive fast or carry much cargo

A lower speed rating or load index can change handling, heat tolerance, load capacity, and the way the replacement set matches the vehicle’s original engineering. Some manufacturers allow specific alternatives and others do not. If the exact original specification is unavailable, use the owner’s manual, automaker guidance, or a qualified tire professional before substituting a lower service description.