I use Fuleio to keep detailed records of my diesel usage since I bought my 2019 Ram 3500 HO.
I had the truck lifted and 37" tires installed. Sometimes I tow a 15k trailer. So, here are 4 different MPGs:
Before the truck was lifted and 37" tires were installed:
Average towing 10.2
Average not towing 15.8
After the truck was lifted and 37" tires were installed:
Average towing 9.5
Average not Towing 14.5
Lifting the truck and installing 37" tires reduced the efficiency by about 1 MPG.
I don't use the truck computer to calculate MPG. I manually calculate the MPG by dividing the number of miles driven by the number of gallons of diesel used.
EDIT: This was measured over 22,000 miles.