6.4 litre HEMI vs Cummins

ShinerBock wrote:

It is speed and load depended. As you can see the BSFC map below. The "sweet spot" for the Cummins 6.7L under load is between 1,600 and 1,900 rpm. The red dots/lines is 6th gear and the four vertical lines staring from the left is my rpms at 60, 65, 70, and 75 mph while what I tow. The yellow dots/lines is my rpms in 5th at the same speeds. The horizontal lines are grade/load starting with 0% grade at the bottom.

On completely flat ground and no drag, 6th is generally best, but even a slight 1% grade and moderate drag will put enough load on the engine to make 5th the better choice which is why I generally leave it in 5th and forget it unless I am going past 72-74 mph.

Thanks for the graph Shiner. I've wondered which is more efficient, but I haven't been able to test it personally. Our regular trip is to a lake in PA, so we deal with traffic and mountains. Too many variables to get a competent comparison. Our yearly Florida trip is about 2000 miles round trip, but I always forget to document my towing mileage separate from my "around town" mileage to compare it.

It makes more sense to have slightly higher RPMs and maybe a little less turbo pressure to get better fuel economy.