stock?

the air filter is oil bath so it could be clogged.

also is it an automatic? they dont do so well on mileage. are you talking around town or highway? if around town 20 is correct.

i dont think the lines will help. and if it is leakig diesel the smell will be obvious. are you sure the odo is working correctly? if it slips then your apparent mileage will be worse.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.