... The MPG follows that band closely too, because the power is coming from the fuel being fully burnt, so if you change gear too early you have dropped away from the optimum efficiency and so it is not strange at all that 4th gear can be more economical than 5th is certain rev ranges and conditions. ...
Yes, I've tried to illustrate this point in the graph here in red (apologies for the wobbly line):
View attachment 19639 (edited)
This graph needs a lot! of explanation because it's trying to display a 3-dimensional data set.
The X axis is RPM, the Y axis is torque (sort-of). The engine load is the proportion between the torque line and the X-axis (i.e., 100% engine load = max torque for a given rpm, 50% engine load is half-way down).
The consumption figures are found on the ISO contour lines, varying here between 197 and 500. You have to imagine that the fuel consumption is on the Z axis poking out of the paper. Higher number = higher fuel consumption per horsepower, so lower is better.
197 is the most efficient point anywhere on the map (1.75k rpm at about 95% engine-load), this is the torque peak that Spike mentioned for this particular engine (the map is for the classic 1.9TDI with a variable vane turbo, not remapped. A simpler turbo would have a torque peak at a higher rpm, say 2200 typically).
The least efficient points are the highest numbers, for example 500 at 4.5k with low load (= revving the engine to the red line in neutral).
Say we are driving steadily at 60mph on the flat, using a steady 30hp to do so. So we follow the blue 30hp line to find the efficiency at particular RPMs.
4k = 320, 3k = around 270, 2.5k = around 245, 2k 220, 1.5k about 204, 1.25k around 208, and the engine can't supply 30hp below that rpm.
The wobbly red line illustrates Steve's point - engine efficiency gets worse as rpm drops beyond the line. So at 30hp, you are worse off at 1250rpm than you were at 1500 rpm. But you are better off at 1500 rpm than 2000 rpm. If you are going up a hill / into a headwind / etc you will need higher hp and therefore you'd be looking at a different blue line.
If you are on the wrong side of the red line this is often called 'lugging' and the engine struggles.
This only takes into account the engine efficiency. Engine load at very low rpm is not particulary good for the gearbox / flywheel even if the engine itself is happy.
Sorry this graph is so tricky, pretty sure that my explanation will just cause confusion lol.