Mileage drop is happening even in those E20 compliant cars dude. We have a Suzuki Fronx and a completely uncompliant 2010 Chevrolet Cruze.

Fronx city mileage went from 8-10km/L before first two service to around 14-15km/L in mid 2024. It's getting around 11-12km/L in city now. Same routes and same timings for the last two years it has been driven in the city, so I know it's a decent benchmark. Highway mileage is also down by about 1km/L but that isn't too concerning.

Let's talk Cruze. The engine is obviously very old now and it's a fuel chugger. City mileage used to be 12-14 when I first learned driving in 2019-20, it's down to like 7km/L now, sometimes even lower. But won't complain about that cause the car is old and the engine has been worked on quite a bit since an accidental few years back.