4x4ord wrote:
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?
This may be where you are also getting confused. Determining what a "light load" is not just determined by the engine power. It is also determined by gearing. A 13k trailer is more on the lighter side for a 4.10 geared truck while it is more of the heavy side for a 3.42 geared truck. Hence the reason that if I had 4.10 gears, versus my 3.42 gears, then I would leave it in 6th because that would put me at the most fuel efficient rpm. My 3.42 geared truck is meant for people like me who do not tow all the time and use their trucks as a daily just like most people who choose taller gears. We are willing to sacrifice towing capabilities, performance, having to put the trans in 5th to be at the most efficient rpm while towing for better fuel economy unloaded.
Manufacturers do not make different trans tunes for different rear gear ratios. YOU are suppose to pick the rear gear ratio that has the best blend of of efficiency/performance based on how you use your truck.
There is a reason why medium/heavy duty ruck makes offer so many transmission and rear gear options even with today's automated transmissions. Because not every vehicle is used the same and there is no one size fits all transmission and rear gear. The customer is suppose to spec these options out based on how they will using their truck similar to how light duty(class 1-3) truck owners have to spec out rear gear or trans option based on how they use their truck. However, we do not get as many options as the medium/heavy truck market does and it is more of a generic spec.
