On my 01 Dodge Ram CTD auto which was modified a bit with rv275 injectors and a programmer set on normal or towing. The truck not towing would normally get 19-20 mpg. That is hand calc'd over 350k miles of checking it almost every tank with summer fuel, winter fuel it would drop about 1-1.5mpg. Towing our 31 foot, 10k fiver it would vary between 10.5 and 13 depending on speed, road conditions and wind. Generally it was right around 12 give or take 1/2mpg
Just got a new 2017 Ram haven't had the trailer hooked up to it for more than a mile so far so cant speak yet about the new ones mileage towing. Not towing so far it varies from 16-19mpg.
