Hi I am new here,
I looked into this same thing for Gas powered engines. I assume it is the same with diesel, but that is a guess
Everything I read states the lower rpm is your best fuel to power ratio. There is more power/torque to accelerate quickly at higher RPM’s. But… The fuel use goes up disproportionately fast.
That disproportionately high fuel burn rate more than eats up any saving you might be getting acceleration faster.
Here is a GREAT article on it.
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After reading up on that I have found my car’s sweet spot for fuel to power ratio looks to be shifting at around 2200 rpm’s. That drops my rpms in the next gear down to about 1850. I have a scangauge I will watch LOD during acceleration, I shoot to keep the engine at around 75 Load.
Any lower RPM’s and I lug the engine, not good for the engine and takes too long to accelerate.
Any High and I definitely accelerate faster but mileage goes down even faster.