Do you practice bird law too ? <| sep |> I don ' t see how it can be the drive train unless the transmission won ' t shift into top gear or the lockup mechanism of the torque converter doesn ' t work , and an engine that ' s worn out and has weak compression will also be less efficient . But Hondas are known to maintain really good compression eve over 200 , 000 miles because of their low tension piston rings . Also because both of your cars dropped about the same proportion , I ' d say the cars are fine and your driving conditions have changed . I don ' t understand how you could have done the factory recommended transmission flush because Honda doesn ' t recommend flushes . If you changed the spark plugs , did you use the factory recommended ones ? Hondas are known to like NGK brand , and at least in 1990 s Hondas , using something else could make the engine run funny but maybe not for a few hundred miles . What ' s the standard deviation of your fuel economy for each ? Of course you recorded all the information needed to calculated that because otherwise you can ' t tell if anything is really out of whack .