UK drivers are being cautioned against filling their petrol and diesel tanks to the brim as a cost-saving measure. A full tank can add around 40 kilograms to the vehicle's weight, which may reduce fuel efficiency by nearly 2% during regular driving. Instead, experts recommend keeping the tank just above half full to avoid overheating the electric fuel pump and to prevent moisture condensation inside the tank. When the tank is nearly empty, humid air can enter, leading to condensation that mixes with the fuel, potentially causing engine damage. Studies indicate that reducing a vehicle's weight can lead to better mileage, making it financially beneficial to avoid a full tank. Therefore, maintaining a tank level above 50% is emphasized as a practical approach for drivers looking to save money and ensure their vehicle's longevity.