Yeah 400 miles a day is a lot. I bought the Duramax with 180,000 on it and it has right at 200,00 already. I have had it 3 months and love it. I average around 21mpg with it. It has aftermarket exhaust and cold air intake on it. It had the 40hp tune in it and it didn't do much different with the tune in it than what it did with the stock tune. Some people told me to boost it to the 80hp tune and I would pick up 3 mpg. I am to nervous to bump it up to that. I am more concerned about longevity of the engine. Is there anything else I can do to bump the fuel mileage? Someone mentioned a different turbo intake. I would really like to get 23-24 mpg