Any car, when driven at lower RPMs and at higher gears will juice out a fantastic average that one can proudly boast of. This can be achieved by gradual but swift shifting from first gear to the top. Remember, a higher gear shift lets you drive faster but it doesn’t necessarily mean that you have to. You can continue maintaining your usual 45 - 55 km/h and engage overdrive without needing to accelerate. The result is a much lower engine speed and, hence, less fuel is burnt.
Many a time, people drive in a lower gear and at a higher speed, causing the engine to strain at higher RPMs before shifting up. This is a wrong practice and must be avoided if you care about fuel efficiency.
In case of automatic cars, it is essential to keep a light foot on the accelerator to juice out a good average as the automatic gearbox in a car will down shift a gear if you press the accelerator more than necessary, which will be a lot of fun no doubt but will lead to really higher fuel consumption.