I got a 3500 Dually with the diesel engine with 3:54 gears,pulling my 40' GN with my dragster,golfcart,a/c,floorjack, battery charger,and tools I have not checked it buy I'm lucky if I get 8 mpg.
This will be a interesting post for me.A few years back when I got my 1st diesel about 9 years ago diesel was the price of the low grade gas now it's above the highest grade gas.