If vehicle sub-systems (engine, gearbox, air conditioning, tyres, etc.) are poorly maintained, your car runs less efficiently and consumes more fuel.
To keep your car running at peak efficiency, you need to have it serviced regularly by a professional mechanic. Make sure that all parts and consumables used (e.g. oil, air filters, tyres) are guaranteed, recommended and tested by Citroën and each component used in producing your vehicle met a full list of specifications.
Engine oil, for example, provides lubrication but also contributes to engine efficiency. This is the case for most of the parts used by your vehicle. A poorly maintained air conditioning system consumes far more fuel. It is in your own interest to keep your vehicle well serviced because it will consume less fuel, be more reliable and command a higher price when you sell it on.