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.
I switched from Nvidia to AMD about a year ago. I was worried because everyone harps on AMD drivers, but they’re pretty solid imo. I have yet to have an issue with them, and I’d say that I even prefer them at this point!