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I wish I was getting 18 mpg. My truck has been getting 14.5-15 mpg ever since I purchased it at 138K. Maybe the compression is down, injectors may not be spraying as good as they can, or something else. Nothing has improved it even driving slower.

I also have had zero luck with additives. I seam to always hand calculate lower fuel economy when I put additive in the tank. I say this based on my experience with a 1994 6.5TD C3500, 1999 Ford Power Stroke F250 and now my 2004 D/A Dually Crew K3500. Normally about 0.5 to 1.5 mpg worse with the additive. You bet, I don't use any additives any more, why spend the money.

I also don't believe the DIC since every single vehicle I have ever owned has told me I get better mileage than I actually get. Current 2004 is 1.5 to 2 mpg better than what I hand calculate. Check your trucks again and base it over several fill ups. This is the only way to really know for sure. My drive is about 60% highway at 70-75 and 40% City. Parts of my drive, I push it to 80 and know that hurts the economy. Average MPH is typically between 38-41 over the entire tank.

Steve
 
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