What is the most fuel efficient air/fuel mixture in an engine?

I’ve heard things that if your mixture is too rich, you won’t get a lot of power and your engine will spit out fuel from exhaust.

If you put a lean mixture, you will waste less fuel but your engine might overheat and receive damage.

What mixture ratio if most efficient in terms of fuel economy without harming the engine?
I realize that 14.7 is the most effective one in terms of power+efficiency, but I’m talking just about the efficiency. I don’t mind my vehicle being slower.