I just have a little over 200 miles on my D90X. Drove it 60 miles back and forth to Miami today, keeping it at around 65-70 and recorded 23.4 mpg for the trip. Short trips around town are giving me 17-19 mpg, which is far better than the 14 I was getting in my 2020 Jeep Wranger Sahara with a V6.

Still waiting for my Auto Stop/Start to start working, even after I've made sure my battery is topped up with a battery maintainer. Although many don't like it, I found in my mild-hybrid Jeep with the same generator starting system that the start/stop functionality was really pretty unintrusive.

 

I'm averaging 21 on my P300 over the first 800 miles. They really ain't bad.

 

I just received my 2022 Disco5 which is a P360. I guess the displacement is the same as your P400, just software tuning difference. I don't expect to get the 30+ highway my old 2017 TD6 D5 got, but these are hopeful numbers.