I have done my own tests like that and I can tell you for some cars weight is good, while others is not. I also have found the best results is using speed control but on steep hills I accelarate a little bit and the cruise control won't shift into the lower gear and I save fuel like this.
You can try it yourself. Put the the car on speed control for 65mph and leave it. When it gets to a hill the cruise control will downshift in order to keep the same momentum, so it will use up more fuel. It generaly increases rpm to 4000-4500rpm to get over a hill.
Next try this. Put the car on cruise control at 65mph. Right before the hill accelarate 5mph more, but do it smoothly or your car will shift into lower gear. So now you're going 70mph and start going up the hill, as you're climbing release the gas really really slow, so as you climb you're losing speed. By the time you get to the top you're at 62-65mph.
The benifit is your Automatic won't switch gears meaning you'll save on fuel and also your rpms will only go up 300-500rpms instead of using the cruise control for going up the hill and it will go up 2000-2500rpm. For both methods you're still on cruise control while going up the hill.
If Matt lets me I can write a great article how to use the cruise control on the freeway increasing 1-3mpg. I don't drive the freeway much but If I did my mpg would be 30-35mpg