What the official roadmap says about E20 mileage
The most quoted numbers are test estimates for vehicle categories, not a promise about your next tank.
Good public information should help people ask a better next question.
The numbers have a test context
The NITI Aayog roadmap reports an estimated fuel-efficiency drop of around 6–7% for four-wheelers designed for E0 and calibrated for E10, 3–4% for two-wheelers in the same broad design context, and 1–2% for four-wheelers designed for E10 and calibrated for E20.
Those figures are useful for understanding the direction and the assumptions. They are not a mileage guarantee for every road, vehicle, fuel batch or driver.
A number is not a diagnosis
The roadmap also notes that hardware and tuning changes can reduce the efficiency loss. SIAM says real-world mileage is affected by driving habits, maintenance, vehicle age, tyre condition and air-conditioning load, among other factors.
A lower tank average is worth recording. It is not, by itself, proof that E20 caused the whole change.
How to make a useful comparison
Compare like with like: the same vehicle and route, similar traffic, tyre pressure, load, air-conditioning use and driving style. Record the fuel grade, litres, odometer reading and receipt for several fills rather than relying on one trip computer number.
If the pattern continues, attach the records to a service request and ask for a written vehicle-specific response. That turns a frustration into a checkable question.