GarageCalc 
Reading the full service description
On a tire marked 225/45R17 94V, the 94 is the load index (1,477 lb per tire) and
the V is the speed rating (149 mph). Together they form the
service description, and both must meet or exceed what your vehicle placard specifies. When
changing tire sizes, compare capacity, not just dimensions: check the size math with the
tire size comparison calculator and make sure the new
size is available in an adequate load index.
Load index 91 vs 94 vs 97
| Load index | Maximum kg per tire | Approximate lb per tire |
|---|---|---|
| 91 | 615 | 1,356 |
| 94 | 670 | 1,477 |
| 97 | 730 | 1,609 |
Index 94 has a rated capacity 55 kg higher per tire than index 91. That does not mean fitting four index-94 tires adds 220 kg of payload to the vehicle. The suspension, brakes, axles and certified vehicle limits have not changed. Also, the load is not necessarily shared equally across all four tires.
Does XL mean I should use the sidewall maximum pressure?
No. A tire's maximum marking is not automatically the correct operating pressure for your car. For an approved original-size fitment, start with the vehicle's cold inflation specification. For an alternative tire size or type, obtain the appropriate load/inflation guidance rather than choosing a pressure from the load index alone. Check pressure cold and account for the manufacturer's loaded-vehicle instructions.
Michelin's load and speed rating guide explains the service description; NHTSA explains placard pressure. When towing, also check the separate limits in payload vs towing capacity. Tire capacity is one constraint, not a substitute for those vehicle ratings.
Frequently asked questions
What does the load index on a tire mean?
It is a standardized code for the maximum load one tire can carry under specified conditions. Load index 94 represents 670 kg, about 1,477 lb, per tire. Adding tire capacities does not establish the vehicle payload or raise its gross vehicle or axle weight ratings.
Can I fit tires with a lower load index than stock?
Replacement tires must satisfy the vehicle manufacturer requirements for load capacity and inflation. Do not substitute a lower-rated tire based only on matching dimensions. Ask the vehicle or tire manufacturer to confirm the exact replacement specification.
What is the difference between SL and XL tires?
XL means extra load and SL means standard load. XL tires can provide additional load capacity at their specified pressure, but the letters alone are not a complete capacity comparison. Check the load index, size and approved inflation requirements; do not replace a required XL tire with SL without manufacturer approval.
What do the load index numbers on dually or LT tires mean?
A dual service description such as 121/118 gives separate capacities: the first index is for single fitment and the second is for dual fitment. Use the rating and load/inflation specifications that match the actual installation.