7/16-20. You'll also need to specify what style nut you need. You didn't specify what brand/style wheel you have, but it can make a difference on aluminum wheels depending on who made them, and how old of a wheel it is. Many older aluminum wheels use a nut with flat washer and shank that fits inside the wheel. If they are that style, you'll need to have the thickness of the wheel face at the mounting points to get the correct nuts. If they are newer style wheels, they mostly use a 60 degree taper seat lug nut, which will make things much easier. One exception to this on modern aluminum wheels is Centerline. For whatever reason, they seem to still use the older style nut.
