some real numbers!
2017 ram 1500, 4 door, with full topper, 8 speed hemi
not towing, 105-110 kph highway driving, we see 9.5-10.5 l/100 km
that's 23 mpg
towing 6500 lb tt @ highway speed
average around 18l/100 km
that's 13 mpg
numbers are on flat road, no perceptible wind, very light traffic
those numbers are from the dash board display but agree very closely with fuel cost over a 6500 km trip
we did see the consumption go right off the scale!! it tops out at 99l/100 km. going up a 17% grade(3 or 4 km long), passing a slow moving car. went from 40 kph at the bottom, accelerated up to 90 near the top in a passing lane. nothing beats the sound of a hemi at wide open throttle at 4500+ rpm and for a couple of minutes!!