What is the most fuel-efficient speed for manual operation in 5th gear?

Under normal circumstances, it should be 2/3/4/5 at 20/40/60/80, and the most fuel-efficient time is when the fifth gear is 80.

Generally speaking, driving at the speed of 70 to 90 kilometers per hour on the expressway is the most economical, the water temperature and oil temperature of the vehicle engine are the most balanced, and the engine wear is the least. When starting in the first gear, lightly step on the accelerator. When the speed reaches the second gear, you can shift to the second gear. Step on the accelerator harder, and you can also change to second gear when the speed reaches C, D, E and F. Different models have different fuel-saving speeds, of course, the most fuel-saving mode is in the top-grade low throttle state. At the same speed of 80km/h, using a 5-speed low throttle is definitely more fuel efficient than a 3-speed high throttle. Generally speaking, all cars will have an economical speed when they leave the factory. According to the same mileage, this is the most fuel-efficient mode.