First, Use the right plug: NGK laser platinum ONLY it runs like crap without them and eats too much fuel with any others ( Yes I know there $10 each, just buy them).
Second, clean the mass air flow sensor, nobody ever thinks of it and it ALWAYS adds MPG. Just make sure you buy the right stuff its called um.. mass air flow sensor cleaner; while your at it put in a new filter.
third, rid yourself of the factory fan and opt for an electric one.
fourth, turn TOD off and install some manual hubs.
I average 18-19 MPG with 20-21 (maybe 22 if theres a stiff wind to my back) MPG freeway, with these mods.
Doc.