Where did the "wheel" group get its name?

The wheel group on nix computers typically refers to the group with some sort of root-like access. I've heard that on some nixes it's the group of users with the right to run su, but on Linux that seems to be anyone (although you need the root password, naturally). On Linux distributions I've used it seems to be the group that by default has the right to use sudo; there's an entry in sudoers for them:
%wheel ALL=(ALL) ALL

But that's all tangential; my actual question is: Why is this group called wheel? I've heard miscellaneous explanations for it before, but don't know if any of them are correct. Does anyone know the actual history of the term?