Consider that an adult human has an average walking speed of about 5.63 km/hr and burns approximately 4.5 calories/minute. (1 cal = 4.184 kj) also, assume that this energy comes from the oxidation of foods, which is about 30% efficient. how much energy do you save by walking 1 km as opposed to driving a car that gets 30 miles/gal? (the density of octane is 703 kg/m3).