E20, Isobutanol and Bio-CNG: What Blending Actually Does to Your Mileage — and Your Fuel Bill

Fuel blending in India has quietly become universal. Every litre of ordinary petrol now contains a fifth ethanol, compressed biogas is being phased into the CNG network from this financial year, and the Road Transport Minister has floated blending up to 15 per cent isobutanol into diesel. Whether that has made your commute more expensive is a question with an actual answer, and it is not the one either side of the argument usually gives.

Three mandates, three different chemistries

The most common error in coverage of this subject is treating all three as one phenomenon. They are not, and the difference decides whether you should expect a mileage effect at all.

This is not like ethanol blended petrol where another substance of differing properties is being mixed into the fuel.

Shreyans_Jain, Team-BHP forum, 7 August 2026

That observation, made by a long-standing member of the forum thread in this brief, is the single most useful thing anyone has said about this subject. Methane from a purified biogas plant and methane from a gas field are the same molecule. Ethanol and petrol are not. So the physics gives you a clear prior: expect a measurable efficiency effect from E20, expect very little from bio-CNG, and expect something in between from isobutanol.

He adds a practical caveat worth keeping in view: “The purification and compression equipment of biogas costs many times more than the basic biogas plant itself.” The chemistry is easy; the economics of getting there are not.

What is mandated, and from when

On diesel, it is worth being precise about how early this is. Nitin Gadkari said “ethanol can’t be blended directly in diesel, so we’re producing isobutanol from ethanol”, and cited pilot work: “We’ve successfully run two generator sets on 100% ethanol and isobutanol. This proves engines can be built to operate on these fuels.” Two generator sets is a demonstration, and “can be built” describes new engines rather than the diesel vehicles already on Indian roads. Nothing in that statement speaks to what 15 per cent isobutanol does in a ten-year-old truck.

The government’s case, in its own numbers

The official position is detailed and deserves stating properly rather than as a strawman. The Ministry of Petroleum and Natural Gas published a point-by-point response to mileage concerns, and its argument is that the programme was never primarily about your fuel bill.

The ministry’s position on mileage specifically is that critiques of a “drastic” reduction are misplaced, because fuel economy depends on driving habits, servicing, tyre pressure and air-conditioning load as well as fuel. That is true as far as it goes. It is also the standard difficulty with any claim of this kind: those same confounders were present before the blend changed.

Petroleum Minister Hardeep Singh Puri put the number at “just up to 3 per cent, which is negligible”. The Automotive Research Association of India has estimated a 1 to 6 per cent dip.

What drivers actually report

The gap between the official figure and owner-reported experience is the whole controversy, and it is large.

The arithmetic that decides your fuel bill

Here is the question that actually matters, and it is the one a reader of this site raised: if mileage falls, you simply buy more fuel to cover the same distance — so does the whole thing wash out?

The instinct is right and the conclusion is understated. It does not wash out. It costs you more, for a reason that is easy to miss.

Why E20 is not cheaper — and why the government does not claim it should be

This is where the reader’s hypothesis gets its answer. If blended petrol carries less energy per litre, the logically consistent outcome would be a lower pump price, leaving cost per kilometre roughly unchanged. That is not what happens. E20 is sold at the same price as the petrol it replaced.

The Ministry’s explanation is straightforward and, to its credit, does not pretend otherwise: ethanol is not currently cheaper to procure. Maize-based ethanol has been bought at around ₹71.86 per litre before GST, transport, storage and depot handling. On the ministry’s own account, blending “is not designed to lower petrol prices on a given day”. It is designed to replace imported crude with a domestic input whose price does not move with Brent.

So the reader’s instinct holds, with one correction. The cost does not stay the same. If there is a genuine efficiency loss and the price has not fallen to reflect the lower energy content, the driver absorbs the entire difference. The government’s answer is not that this is untrue — it is that the benefit is national rather than personal, and shows up in the import bill rather than at the pump.

Whether that trade is worth it is a political judgement, not a technical one. But it should be made with the arithmetic visible, and the arithmetic is above.

The market has cast a vote of its own

There is one number that summarises consumer sentiment better than any survey. The Maruti WagonR BioFlex, India’s first mass-market flex-fuel passenger car, capable of running on blends from E20 to E100 and homologated for E85, launched on 4 June 2026 with deliveries from 17 June. In its first weeks it recorded three registrations.

It carries a premium of roughly ₹74,000 over the equivalent top-spec petrol variant, at ₹7.24 lakh ex-showroom. In fairness, industry commentary attributes the figure largely to the absence of E85 pumps rather than to hostility toward the technology — a chicken-and-egg problem in which retailers will not build dispensing infrastructure without vehicles, and buyers will not buy vehicles without fuel. Both readings can be true. Neither is an encouraging signal for voluntary adoption.

What could not be verified

Final Verdict

Three things are true about fuel blending in India at once, and most commentary picks one and stops.

The government’s macro case is real and largely uncontested on its own terms: ₹1.44 lakh crore of foreign exchange conserved, 245 lakh tonnes of crude substituted, and a genuine transfer of money that used to leave the country toward domestic agriculture. You can dispute the emissions accounting or the sugarcane water footprint, but the import substitution is arithmetic.

The driver’s complaint is also real, and the government’s own framing concedes the shape of it. If your mileage has genuinely fallen and the pump price has not, you are paying for a national benefit out of your own tank, on a schedule nobody asked you about. At a 10 per cent loss that is roughly ₹8,900 a year on the assumptions above; at 3 per cent it is under ₹2,500, which is closer to noise than to a grievance.

And the three fuels should stop being discussed as one. Bio-CNG is purified to be substantially the same gas that is already in the pipeline, and there is little physical reason to expect it to behave differently. Ethanol genuinely carries less energy per litre. Isobutanol in diesel is not yet a consumer product at all. Lumping them together produces the worst of both errors — unwarranted alarm about bio-CNG, and unwarranted complacency about the one blend that has actually changed what comes out of every pump in the country.

The practical advice is narrow but useful. Measure your own mileage across three consecutive full tanks before concluding anything, because a single tank tells you about traffic rather than fuel. Check tyre pressure and service history first, since both move economy by more than 3 per cent on their own. And do not pay a premium for higher-octane grades in the belief that you are buying less ethanol, because with the single exception of 100-octane, you are not.