I just bought a new 2004 Duramax Chevy, extended cab with short box. I filled the tank at 226 miles and showed 17.2 mpg. I filled again at 500 miles and now show 15.2 mpg. This is kinda discouraging. Is this normal? I bought the D/MAX expecting 18-20 mpg. The only difference is a tonneau cover being added between fill-ups. I drive 25 miles to work on four lane highway at approx. 70mph. Any comments??