Has anybody any ideas how to increase gas mileage on '97 SC400?

Increase the tire pressure to the max as indicated on the tire: This will give a harsher ride though, and it will increase your stopping distance.
Ease up on the throttle, don't fly from standing still.
Made certain that your gas filter is changed. Solid deposits (sediment) in the gas tanks can get into the fuel system of the car, with a clean gas filter your car should be able to filter these elements out of the system for a cleaner burn when the gas reaches the engine for combustion

Has anyone actually done any modifications to try to boost the gas milage, such as computer tuneing, mass air flow sensor, cold air intake, exhaust modifications, etc?

Anyone can get better mileage out of their vehicle. It’s not magic. You just need to apply a little technology and a good portion of willpower. And with today's high gas prices, it's worth taking a few minutes out of your day to make minor modifications that will improve your gas mileage. Visit this one, newcarbuyingguide.com/index.php/news/main/1880/event=view

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accuratesignings

If I am not wrong than your suggestion will increase the gas mileage for every vehicle, right?