Fill its petrol tank to the brim
Drive on motorway for a 100kms at a constant speed of 100kmph
Refill from same pump till brim
Then calculate mileage.
If it turns out to be less than 15 only then start to rectify, instead of just throwing in parts.
If motorway travel not feasible measure your ‘Average speed’ while calculating fuel averages. Average speed is the single most important determinant of fuel average.