Biggest drop in diesel MPG is the fuel they are suppling us. This new low sulphur fuel has less power and lubeitricity. Off road and home heating oil will delivery better economy, but is illeagal. A Chevron employee stated one of their additives was nothing more than ATF, but the cheaper ATF over additive has red dye in it. Sweet crude can be run in a diesel with excellent results[ caution some wells produce better grades than others] Crude can be mixed with diesel fuel. Make sure it is filtered. I have boosted my MPG by 4-5 mpg with a mix 325 gal crude 50gal diesel fuel. Also adding cooking oil can help, but cold weather can thicken it. Basically get the air in and out of a diesel for the best economy. High flow air filter [KN, Air rad, ect] plus they last for a life time just clean them, less restrictive exhaust. Clean the injectors I've used Lucas additive with good results put qt. bottle in engine compartment and ran suction and return line into, or not as good just fill new fuel filter with it and run it.