Also need to take into consideration the type of gearbox fitted into the car i.e. CVT, torque convertor etc.
One thing I noticed yesterday when towing the caravan back from Somerset and travelling at a lower speed is that the car seemed to change gear more often from a high to a lower gear which then amounts to higher revs and more fuel used.
I am not sure however if this balances out i.e. towing at a higher speed less gear changes, but using more fuel or travelling at a lower speed, more gear changes and using more fuel on gear changes?