Grade 5 titanium. 40x 12point 5/16-uncx1.25″ & washer
Titanium weighs 45% less than steel. If your car has 10kg of nuts and bolts aboard, you could potentially save 4.5kg.
Titanium has a similar strength to steel, despite its lower density. This makes it a preferred choice for high performance applications including motorsport and aviation.
A little known benefit of titanium is its resistance to corrosion - no fighting rust. Make your high performance application last.