Fit it safely
Park safely and switch the engine off. Find the OBD2 socket in the vehicle handbook, inspect the connector and make sure the surrounding footwell is clear. Align the Fuel Saver plug and press it into place with gentle, even pressure. Do not force a connector that does not fit.
Use the stated settling period
The product information gives a period of about 240 km. Continue normal driving and keep a record of complete fuel fills. Compare motorway, urban and mixed routes separately because traffic, temperature and load can change consumption.
Read the results consistently
Calculate UK mpg by dividing miles travelled by litres used and multiplying by 4.54609. Calculate litres per 100 km by dividing litres used by kilometres travelled and multiplying by 100. Several comparable fills provide a clearer pattern than one trip.