theoretically the most fuel efficient would be not having it on at all.

But it really depends on how vehicle refrigerant phase change loops work. I’m not a mechanic nor am i familiar so hopefully someone else can chime in?

There are a few outcomes here depending on the cycle. If it uses some sort of refrigerant flow restriction that is variable, then theoretically the thermostat setting will have an effect. (i would assume this is a case these days) However the effect it will have is not going to be linear, given that its a fluid pump cycle. Though it’s powered directly by the engine (most likely?) so that probably won’t matter at all, engines are horribly inefficient to begin with.

if it’s not using some sort of variable valve to control restriction and thus cooling potential, it’s doing some other shenanigans and running the phase change at a set point, though it would have to introduce some sort of external variable there in order to compensate so probably not.

I would probably set the ac to a low temp, and then the fan to a low speed, because it takes advantage of physics better that way (the greater the difference in temperature, the more effectively you can transfer heat away from it) since you’re running a low fan speed, you’re gonna need a high temperature gradient to make the best use of that. There is also the inverse, but i doubt that would be better tbh.

Though i think realistically, it just won’t matter enough for you to bother. Like you’ll probably see a bigger difference in usage by taking a different route somewhere, or idling longer. There are a number of other factors that can impact fuel efficiency.