That depends on the driver. A driver who is conciously trying to save fuel will always use less fuel than a computer which is trying only to maintain a constant speed. A driver who is "just driving" probably won't
One of the big differences is on hills. A human will naturally tend to let their speed creeep up going down hill, getting "free" speed and then let it slowly bleed off going up a hill whereas a computer will back off the throttle going down the hill then bury it going back up, using more fuel.
On flat roads, it's likely the computer will beat the human