The government's own report predicted your mileage drop. In June 2021. 4 years before the nationwide rollout.

What the report admitted in 2021: Older cars lose 6 to 7% mileage on E20. Older bikes lose 3 to 4%. Its own experts recommended keeping E10 available for older vehicles and giving a tax discount for lower energy fuel. Neither happened. No label at the pump. Nearly 23 crore older vehicles moved to the new fuel without being told. When a citizen filed an RTI asking for the testing reports, the answer was: confidential.