I have a 2002 Silverado 2500HD with 6 liter and 4.10 rear. Towing my previous 8500+ lbs trailer at 60 mph, I got between 9 and 10.5mpg depending on flat and level or mountains. Now towing my new trailer around 5000 lbs., I get 10.5 to 12.0mpg at 60 mph. 65 to 70mph equals less, 50 to 55mph, fractionally better. Got 156K on it and still runs fine with no engine or transmission problems YET. Noticed if you blast away from signals or passing gear up steeper grades all day, you will get lower mph.