Is it the most fuel-efficient to keep the engine speed within the maximum torque range?

This statement is too one-sided and theorizes the actual road conditions. The maximum torque speed is not a specific specific speed, but an area, such as 2500 = = 5500 rpm. The faster the engine speed, the more fuel injection times and the more fuel consumption.

This is theoretical, but in practice, for example, when a truck climbs a big slope, the situation changes. If you can't keep the engine running between maximum torque and speed, then you have to downshift to a lower gear, which will save more fuel. At this point, the engine throttle is fully open and fully closed.

However, if it is downhill or on a straight road, it is not comprehensive to keep the engine at the maximum torque and speed to save fuel. At this time, the engine speed is either within the maximum torque speed range, but the throttle valve has no opening or the opening is very small. At this time, the engine is driven by the chassis speed, which means that although the engine speed is high, there is no load.

Therefore, it is not the speed but the throttle opening that determines fuel consumption.