On a flat road, the wheels are more efficient. At least because of the smaller number of moving parts and as a result of lower friction losses.
Most likely the minimum required number of legs is two. We then go on two constantly balancing the center of mass, which we have higher. You can do three and four and six, there is a video with such robots. More legs - more smoothness, you can limp by damaging one of them. But the fewer legs, the less frictional losses in the joints, and the cheaper the "walk-car".
Why not do it? They make robots, robots move smoothly, animals and insects feel good. But it is probably much more efficient to build a flat road and ride on it on wheels.
Anyone can take a scooter and test the effectiveness. Push off with your foot and measure the distance that will roll. Then repeat the experiment without a scooter, push off with your foot and take a step.
But on rough terrain, it is better on feet than on wheels. Because you have to choose where to put your foot and where not to put your foot. In the case of a wheel, it rotates inseparably from the surface, and therefore, in the event of a collision with a stone, it is necessary to raise the mass upward so that it cannot fall through the stone. And this means a greater loss of energy than in the case of walking, because then the center of mass can not be raised.
A bus on feet or on wheels? (I supplement the question due to the lack of answers!) What is more efficient, a bus on feet or on wheels? You can take as an example not only a bus, but also a car or even a motorcycle and others. How many robotic legs and what power and other characteristics? What is the ratio? How would robotic legs be more effective than wheels? (Maybe for difficult terrain) Will there be passenger shaking and how to compensate for this to a minimum? What algorithm for rearranging the legs should be? I have a bunch of questions and I would like to get an answer! What prevents you from creating a transport with legs? I would like more technical details from you in the answers! And where does each of the options win or lose?