Automobile tire bolts are generally made of high-strength steel. The strength grade of the screw is printed on the head of the tire screw, including 8.8, 10.9 and 12.9. The higher the value, the higher the strength. Generally, bolts with strength of 8.8 or above and heat treated are called high-strength bolts. The hex bolts and nuts of cars are high-strength bolts, and the strength of matching bolts varies with different models and load capacities. 10.9 is the most common, 8.8 is generally matched with low-end models, and 12.9 is generally matched with heavy trucks.