Well, a human certainly *can* drive a lot less efficiently than cruise control. But a human can very easily drive a lot more efficiently than cruise control, because cruise control does some really wrong things.
To increase fuel efficiency, you can coast on the downhills (neutral is better, but switching back and forth into neutral puts more wear on your transmission so that maybe you'll need to replace it twice during the life of the car instead of once, so it's a tradeoff that depends on other stuff), use the flats and/or downhills to gain speed, and slowly lose speed on the uphills. Do not keep a constant speed, let it vary up by to 15mph (so in a 65 zone, maybe vary between 55-70), use less gas when going uphill. Keep a long enough distance between yourself and the car in front of you that you can avoid ever needing to break except in really dense traffic.
Re: Scion, and cruise control
To increase fuel efficiency, you can coast on the downhills (neutral is better, but switching back and forth into neutral puts more wear on your transmission so that maybe you'll need to replace it twice during the life of the car instead of once, so it's a tradeoff that depends on other stuff), use the flats and/or downhills to gain speed, and slowly lose speed on the uphills. Do not keep a constant speed, let it vary up by to 15mph (so in a 65 zone, maybe vary between 55-70), use less gas when going uphill. Keep a long enough distance between yourself and the car in front of you that you can avoid ever needing to break except in really dense traffic.