I'll get straight to the details pretty much, 99.5% of my driving is highway driving, I live off a highway, my work is off a highway which is 2 hours from my house.
Speed limits are 60km/h, 80km/h and 110km/h.
What I've noticed is,
P) control at 60km/h jumps between 4.9l/100km and 12.0l/100km
At 80km/h jumps between 3.6l/100km and 8.6l/100km
at 110km/h jumps between 3.2l/100km and 6.8l/100
However, and this is easy for me as I've just upgraded from a car with and pedal controlled throttle body, that if I drive the car all on my lonesome without any cruise control, in gear 5 I can sit comfortably at 2.9L/100km while keep speed, which is actually huge considering you're not spiking and demanding more fuel for little inclines etc...
I'm assuming GM has a rather sensitive throttle?
This was on a turbo diesel 2010 2.0L holden cruze by the way, 35L out of the 60L tank has gotten me 777.5km and that was using all cruise control, I think I could see an increase of atleast 35% doing manual throttle control.