2024 Fuel Mileage 6.6 Duramax Diesel. Here's the average duramax diesel mpg you can expect for popular trucks Still, an average of 13 to 15 mpg is pretty respectable and is on par with gm’s, chevy’s, and gmc’s rivals from ford or cummins.
6.4 towing 8,000# ball tow rv. 300,000 miles is high mileage for a 6.6 duramax, but they can go much further if taken care of.