$300 will buy you about 100+ gallons of gasoline (depending upon the cost of gas in your area) If you purchase anything that truely improves your MPG it will likely only be 1-2 MPG. Unless you put a lot of mileage on your vehicle, it would take you a very long time to recoop your $300 expenditure.
The X-Caliber tuner can provide a slight increase in mileage, and a HP boost as well, but if you want to use the HP, you will still pay for the extra gas that those extra HP use.
The best way to save gas is to use the free things like keeping your tires inflated. Don't use the truck for short trips of less then 5 miles. Accellerate slowly, and stop gradually by anticipating the traffic lights. Keep the engine tuned and well maintained. And avoid hauling a lot of extra weight around.
You might get a slight benefit from changing to a fully synthetic engine oil, transmission fluid and rear axle gear oil.
Smaller lighter vehicles are more fuel efficient, and the Sport Trac like other EUV's are not designed for high gas mileage.
...Rich