Keys to saving fuel with an SC400....
1) Inflate the tires just slightly beyond the recommended tire pressure limit, and allign all the wheels.....When the car rolls easy the fuel will be saved..
2) Remove your spare tire, tools, and jack, along with your 12CD changer and any other weight in the car you can think of. Also go on diet yourself...Less weight means less fuel will be used..
3) Install a piggy back ECU and then re-tune the engine to run lean... That will save you at the pumps and maybe even give you some more power to boot..
4) Tune the car up - replace plugs, wires, battery cables, fluids, etc.. A good running motor will be more effecient for fuel economy.. If the engine works more efficient it should move the car easier and save you fuel..
Good luck that's all I can think of...