The most efficient driving speed is about 40-45 MPH. This is the point where the exponentially growing aerodynamic drag begins to require much larger power to overcome and thus hurting fuel economy. Tire inflation, alignment settings, and most importantly driving style will affect your ultimate mileage but the maximum efficiency is not too hard to calculate. Since you can't drive 45 everywhere you go then increasing your driving efficiency i sa good way to save gas. 65 vs 75 will make a difference - accelerating smoothly and coasting to a stop instead of running up and slamming the brakes will also help. Tire inflation as mentioned can cost as much as 10 percent. Rid your trunk of any cast iron that is just bouncing around also.
With only a small amount of attention I'd think that most drivers could save at least 10 percent - maybe more if you are a tailgating, on the gas, on the brake, always hitting it at the stop light kind of driver.