Impossible is the only correct answer. Sorry. You cannot change the laws of physics. I don't know if it was faulty recording, or a faulty measuring device but getting better fuel economy at 75 than 55 is impossible unless perhaps you had it locked into second gear at 55.

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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.