as far as I know, it depends
Around here every car has waaaay more engine (and bulk) than it needs for efficient cruising. So things that detune the engine (i.e. hot air intake) can be effective.
But on the other extreme, you can make a vehicle more efficient by making it lighter and put a small engine in it, with turbo for even less weight and displacement. So that they incorporate more performance type mods (big bang?) because they are tiny to begin with, and CAI would be a good candidate in that scenario. But this is not a common machine in my experience. I think my 250cc motorbike comes close, but even it is a compromise between being able to cruise efficiently and being able to accelerate (it is a bit large for most efficient cruising IMHO).