Change how you drive
The biggest impact you can make on fuel consumption is by changing your driving behaviour.
By accelerating and braking smoothly you can considerably reduce your fuel usage. Aggressive high revving acceleration burns far more fuel than a steady increase in speed. Likewise, if you brake suddenly, it just means you keep revs higher for longer than by removing power and slowly decreasing your speed. So always look ahead to make better predictions for your driving.
Keep a steady speed by using the cruise control function when on longer stretches of road such as motorways. Adaptative cruise control, available on many modern cars, keeps your speed and revs maintained and steady.
Avoid speeding, as the higher the speed, the greater amount of fuel is burned.
Even small changes can make a worthwhile impact over a long period of time.