A fuel efficient vehicle travels farther for a given amount of fuel.
Joy efficiency is the degree of joy one is able to harness from a given amount of material possession. A toddler bubbling in delight with a pile of sand is joy efficient. So is Mahatma Gandhi who led a full life despite deliberate poverty. On the other end of the spectrum is the billionaire who dispatches disgruntled tweets from the comfort of his private jet.
Fuel efficiency is a feat of engineering. With joy efficiency, we can even further. While fuel efficiency is governed by the rigid laws of thermodynamics, joy efficiency, like the human mind, knows no bounds.
Observations 