Highly dependent on the vehicle.
For most vehicle it does, but I’ve had a few (like, half the cars I’ve owned) where the gearing and the power curve of the engine were/are such that it doesn’t. For instance, my current car only has ~70HP so if I hit any sort of incline in overdrive or exceed ~50 MPH then I have to put my foot into it enough that I use more gas than I would if I just kept my foot were it was, disengaged the O/D, and allowed the RPMs to rise from 2200 to ~3500.
Diesels are biased more towards low-RPM torque production, and most modern cars have enough power that they don’t labor at highway speed on level ground, but it’s literally true that your mileage may vary.