NEW DELHI: How much is the estimated drop in fuel efficiency when E20 fuel is used in non-E20-compliant vehicles? While govt rejected claims on social media of substantial loss in mileage due to E20 petrol, transport and petroleum ministries have given slightly different figures.Road transport ministry, citing a study by Automotive Research Association of India (ARAI), Society of Indian Automobile Manufacturers (SIAM) and Indian Oil Corporation, informed Lok Sabha on Thursday that fuel efficiency of BS-III, BS-IV and BS-VI vehicles designed for E10 petrol “may be reduced by 2-6%, depending on the vehicle category and vintage” when operated on E20 fuel.On July 20, petroleum ministry had told Rajya Sabha that “any reduction in fuel economy in certain older E10-designed vehicles is generally limited to about 3-5%.” It added that E20 petrol offers a higher octane rating, superior anti-knock characteristics, cleaner combustion and smoother engine operation.Both ministries have maintained that fuel economy depends on several factors, including driving conditions, driving habits and vehicle maintenance.