I think there are various principles at work. One principle is the idea that an engine is most efficient at the rpm where it makes peak torque. Another principle is that throttling down and gearing up under light load conditions conserves fuel. Shiner is pointing out that another principle is that small diesel engines run most efficient around 1800 rpm under medium to heavy loads. I have trouble believing that it is complicated for engineers to write the shifting software for our trucks’ transmissions to ensure that the engines run at the most fuel efficient rpm under light load conditions. (our engines shouldn’t be lugging in 6th at 1100 rpm when they are coupled to an automatic transmission) Maybe the questions that need to be answered are: what is a light load? Is it different for a 300 hp 6.7 litre engine vs a 475 hp 6.7 engine vs a 525 hp custom tuned 6.7 with a stock turbo?
