I get better milage when I add some SVO to my tank. Right now I'm running 1-2 gallons of virgin veggie to the 18 gallons diesel. It runs better and I get 27-ish. If I don't add the veg, I get around 25. And this is in the winter with #2 fuel.

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Ginny in Denver-ish
78 300SD, 265K (mine)