This is not a very well known option since it was new on the 2017 full size GM vans.
This vehicle is used to carry heavy loads typically running about 7500 lbs Gross weight, but can handle up to 8600 lbs. We currently have about 6500 miles on it.
Actual MPG Figures (running loaded at about 7500lbs)
City 20 to 21 (in city of Chicago)
Highway at 60 mph, a little over 27MPG
Highway at 70 mph, 30.3 MPG
The 8 speed Allison trans does not shift up to 8 gear until 67 mph, so the engine runs about 2100 rpm at 65, and about 1500 rpm at 70
This mileage is about double the 6.0 L gas v-8, yet this little engine can really get this monster vehicle moving around town since it develops about 325 lb ft of torque at only 1500 rpm ( about 100 HP), about the same as the 6.0 but at lower RPM. ( both engines are about 370 lb ft peak torque)
This is the same Diesel in the colorado/ canyon pick ups, but they only have a 6 speed. From what I read, we are getting about the same mileage as the pickups, but with several thousand lbs more weight and much bigger drag.
If you need to haul a lot of weight all the time and be able to tow trailers with a full framed vehicle., there is finally a really good alternative. Payback due to fuel savings for the optional diesel is only about 18 months when driving about 20,000 miles per year.
Those that have heard about it are already starting to clamber for the engine to be installed in the full size pickups. This engine will be probably destroy Ford's turbo v-6 truck market, since the six gets horrible mileage when carrying heavy loads and is only rated at 24 mpg highway.