I would say the slowest speed you can do comfortably in top gear will be most efficient.
I'm inclined to agree with that, though I've heard some people claim that being in a lower gear doesn't harm your fuel economy. Surely you're wasting energy purely by making the pistons go up and down more often than they have to. Or maybe that's just my poor grasp on physics.
I've found noticeable differences between cruising on the motorway at 80, at 73 (which is 70 according to GPS) and again at 70. Each time, there's a marked improvement. I once tried resetting the trip counter in a set of 40mph roadworks and let it lug along in 6th gear at about 42mph on the cruise display (40 on the nail according to GPS.) That's about as slow as you can go in 6th without it complaining and I was getting 80+MPG on the level!
Typically, the car achieves high 40's at my usual motorway cruising speed.
So yes, I'd stick with 'The slower the better as long as the car is in top gear and the engine doesn't feel laboured.'
There's also fuel to be saved in the old trucker's trick of 'draughting' (riding the slipstream of the truck in front.) I've used this trick a few times when I've been low on fuel and too lazy to fill up, and I've managed to get very high 40's out of my old 2.0
that used to struggle to break 30MPG. But draughting is risky, and to get the full effect, you have to seriously compromise your stopping distance. As I said, I've only ever used this as a 'get me home on fumes' measure, and I'm not particularly proud about doing that
I'm not recommending that anyone does this habitually to save pennies.