I agree with some points above, give the car some welly to loosen it up, it will cost in fuel for a few weeks but you will reap long term benefits.

The main thing to do is look at your own driving.

The golden rule is all ground covered whilst your foot is off the throttle but not braking is free, so drive to the traffic conditions and terrain, use downhills to help the car accelerate and coast, try not to slow up approaching a steep hill let the momentum assist the climb, if you see a junction or other hazard requiring a slower speed ahead allow the car to coast up to it without braking from as far as needed, try to drive without braking if possible.

The biggest enemy of economy is your brakes, if you have to use them other than in the gentlest of applications then you did not anticipate well enough what was happening ahead...learn to look as far ahead as possible and pre plan as much as possible, it sounds like hard work but once you get into the habit it becomes easy.

Do not tailgait, leave a good distance between you and the car in front, chances of that car being driven with high anticipation skills are remote, watching other drivers constanty accelerating then braking whilst you simply ease along on a semi trailing throttle without going near the brakes will show you the difference.

Find the best pulling revs for your car, try to keep it below 3k RPM once its run in and avoid full throttle unless you need it for safe overtakes and exits from junctions etc.

If your car has an on board computer try to put the instant fuel consumption reading (if it has one) permanently on....that reading will help teach you how to drive for best economy, the funny about this is that your journey times will barely change overall but your driving becomes far more pleasurable.

Some of this is how i drive my lorries (except keeping the revs a bit lower, (between 900 and 1300 rpm), the nett fuel gains are 15 to 25% better than others with identical vehicles.

One other thing, do check the brakes are not dragging, will the car run free?