Efficiency, yes. The second part is a little misguided though. The different gears in a transmission allow the vehicle to move at your desired speed while keeping the engine’s speed low, thus reducing fuel consumption.
Power to fuel consumption isn’t really a thing afaik. Naturally, a slower spinning engine will use less fuel.
Go ahead and live with dentures then. When your bottoms pop out every time you eat steak, remember what you said here. Teeth are absolutely essential. It’s cheaper to take care of your natural teeth than it is to drop 30k on implant-supported dentures, or if you’re broke, just basic ones.
Source: I work in the field. Bottom dentures are by far the most disappointing service we do.