On our 05 chev the dash showed the correct fuel mileage. Math was the same result. Then in 09 on the trip to Wisconsin the speedometer stopped working as did most 05 chevs. After it was fixed back home the mileage and fuel used were way off. I had a reflash done last summer and both are good enough but not perfect as once. So I feel other brands can do the correct fuel mileage as ours did but the math will be the deciding factor. After talking to many diesel guys pulling the same weight as we do or close to it we all get about the same mileage. As we know the roads and winds make up what we get.
chevman