Ideally an engine should be run at the rpm where maximum torque occurs, typically around 2,000 rpm on modern turbo engines but often disguised by flat torque curves, eg 1,700-2,500 - but some petrol engines, often Japanese, have maximum torque up at 4,500 rpm which is difficult to get economy from.When we had a 6 speed manual I often found 5 th was better for towing at 55/60. The rpm was higher but the engine didn’t sound laboured. I believe an engine in a higher gear , lower rpm needs a wider throttle which may use more fuel