interestingly i've found that there are apparently 2 "plateaux" for fuel economy on the v6 3.0, this applies equally to my sintra and miggy, one at around 65-70, and another at about 92
Yes I can definitely believe that, one of the biggest issues with economy with petrol engines is pumping losses due to a mainly closed throttle, this is one of the reasons diesels are good for economy. Pumping losses will peak during part throttle full load conditions so sometimes it is more economical to change down a gear and open the throttle almost totally, enough to reduce pumping losses but not enough to activate the ECUs full throttle enrichment (the equivalent to an old carbs accelerator pump) which after a bit of practice becomes second nature.
I suspect there is a further plateaux lower down though, 65-70 seems quite fast to me for economical driving

(I can't believe I just said that, my other car is a modified trackday 205 GTI that I never drive slowly or economically so this is all new to me!)
meaning one gets better econmy with cruise at 92 , than at 85.
You can get MUCH better fuel economy without using cruise control, its better than normal driving if you aren't thinking of economy but its far from efficient. The main issue is it will accellerate up hills then back off on the way down which is the opposite of what I would do.