Generally the most efficient is highest gear-lowest RPM, and with car makers that's usually around 55-60. You could change the fuel efficiency to different speeds(I'm sure they're adapted to local markets with metric vs Imperial), but it would be worse off for most people to do that. It's not simply a gearing issue to change fuel efficiency, it's also the ECU controlling the fuel/air going into the engine. They aim for optimal fuel efficiency for normal driving, and that seems to be the sweet spot.
Contrary to what I've heard before, changing fuel efficiency to higher speeds wouldn't make things better. It's also not that simple because drag increases exponentially with speed.
Contrary to what I've heard before, changing fuel efficiency to higher speeds wouldn't make things better. It's also not that simple because drag increases exponentially with speed.