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.