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I have found that on flat terrain, holding a set engine load (with reference to the MAP reading) works for a while, until I get down around 52 mph or so, then it takes the same amount of load to hold the vehicle at that speed. The TC locks at a very low RPM on my car in 4th gear (around 1450 at 55 mph), anything below that speed and the engine just doesn't seem to have to torque to accelerate back up to cruising speed efficiently. Adding just a little throttle brings the inst. MPG well below where it should be, and the engine lugs at this speed as well. It seems that 55 mph is just about the minimum that the engine produces efficient torque with the TC locked.
On the other end of the spectrum, anything above 55 seems to be inefficient due to drag and resistance, as it takes a higher engine load to maintain speed here as well.
So it seems that if I want to cruise at a constant speed on level terrain, cruise control set at 55 is the way to go. I don't really think very much efficiency is lost by varying engine load up and down at a set speed. We know that engines are more efficient at high load in the lowest rpm that produces the most effective torque, so accelerating up hills in 4th gear with the TC locked should not produce any loss of efficiency. Rather, the loss is in the engine braking action due to pumping losses as the car coasts down the hill in-gear, or even adds fuel to maintain speed when none is needed. I have found that backing off the throttle and letting the TC unlock on light descending inclines gives higher numbers than trying to maintain a specific load and keep the TC locked.
Pulse & glide with the engine idling and trans. in N seems to work quite well on both hilly terrain and level terrain, especially between 60 mph and 52 mph. So my new strategy will be to practice P&G EIC on hills and when practical wherever else, but to set the cruise at 55 in traffic or when not wanting to P&G. We'll see if this does anything to my avg mpg over a few tanks.
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