But then what about "optimal car-buying"? Surely if someone is taking the time to describe how to buy a car, they probably think it's the optimal method, or at least as close as they can get. So "optimal" would seem to be redundant too, and yet we would seem to prefer one over the other, even though they basically mean the same thing thing in this context.

Now, there may be some arrogance built into "rational" that's not present in "optimal," but I don't see the issue as one of redundancy. Rather, it seems like "rational" can sometimes come off as an assertion of superiority over another -- i.e., something like a man telling a female colleague that she needs to be more rational.