Cars Q&A

In MUmbai we get only the new petrol for BSIV compliant cars. I used to get 12.50-13 kms per litre on HP Power petrol for my ZXI Dzire(BSIII) prior to April 01. Now the mileage has dropped to 10.5 to 11 with the new petrol. My car is BSIII version. No change in the route or my driving patterns. Hence the question by buying high octane petrol meant for BSIV version cars, should the mileage drop? Any expert views will be helpful. Thnak you Rajan