-I've always tried to run my tires about 5 psi over the 35 recommended. Make sure to keep them aired up all the time.
-If you don't use them, take off the roof racks.
-Use an energy conserving motor oil
-When taking off from stops, keep it under 2000 RPM, though every so often, it's good to go up, to keep carbon deposits from forming
-On the highway, try not to go above 60-65
-When coming up to lights, try to coast to a stop instead of getting there quickly and hitting your brakes
-If possible, run your AC in vent only. If you must use AC, use Max AC instead of the just AC since it recirculates the cold air
-Keep your interior clean so as to not carry unnecessary weight
-Make sure your alignment is set straight. At the very least, it'll save on tires
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Those^ are all the free ones. Of course, you could spend money and get e-fans, underdrive pullies, intake, exhaust, etc, though usually people get the for extra power. Mileage is a nice side effect, assuming you can keep the right foot out of it
