Just to clarify a little, the wheel bolts are indeed subjected to tensional forces as that is what holds then onto the hubs, but the more critical forces will be the shear forces that the wheel rims will put on the bolts as the wheels will be trying to "shear" through the bolts when the wheels are spinning.They would most likely be grade 8 which is a hardened steel, and have a UTS of 64kg/mm2, but as wheel bolts are in "shear" rather than in tension this wont be too critical.