Most drivers focus on tyre size (e.g., 225/45 R17), but the load index and speed rating are equally critical from a structural engineering perspective.
Load Index – Structural Load Capacity
The load index corresponds to the maximum weight a tyre can safely support at its rated pressure.
For example:
- 91 = 615 kg per tyre
- 94 = 670 kg per tyre
To calculate required load capacity:
Vehicle Gross Axle Weight Rating (GAWR) ÷ 2 = Minimum per tyre load requirement
Exceeding rated loads increases:
- Sidewall deflection
- Heat generation from cyclic deformation
- Risk of ply separation
- Accelerated shoulder wear
Modern radial tyres use:
- Polyester carcass plies
- Steel belt reinforcement
- Nylon cap plies (in high-speed tyres)
Improper load index reduces structural margin of safety.
Speed Rating – Heat Resistance & Structural Stability
Speed ratings (e.g., V, W, Y) indicate maximum sustained speed capability under load.
Examples:
- V = 149 mph (240 km/h)
- W = 168 mph (270 km/h)
- Y = 186 mph (300 km/h)
Higher speed ratings require:
- Advanced rubber compounds
- Reduced hysteresis
- Stronger belt package bonding
- Enhanced tread block rigidity
Using a lower rating than manufacturer specification:
- Voids insurance in some cases
- Reduces stability at motorway speeds
- Increases risk of tread delamination
Always match OEM specification found on:
- Driver door jamb sticker
- Owner’s manual