It's all in the BTUs. If you have an efficiently running engine, you will squeeze all of the available BTUs out of your diesel#2. If you add anything to that fuel (other than BTU producing fuel from another source, say BIO) you are not adding any BTUs, so are not contributing to mileage. When you add something like BIO, you will alter the average mileage, according to whether the added BIO produces more or less BTUs per gallon than the fuel it displaces. So far, no additives have been made (BIO included) that will improve BTUs per gallon over plain ordinary (clean) Diesel #2.
