India's decision to increase the share of biofuels blended with petrol is facing ire from motorists who question its viability in a market where a majority of vehicles are not yet designed for it.
The world's largest two-wheeler market and third-largest car market, is trying to reduce its dependence on imported oil and cut carbon emissions. It began blending ethanol - made from crops such as sugarcane and maize - into petrol in the mid-2000s and has steadily increased the proportion ever since.
Complaints intensified in April this year when India made E20 - petrol blended with 20% ethanol - the standard fuel at every pump, replacing the 10% blend that most vehicles are designed to use.
Although unblended petrol is still available, it is often 40-50% more expensive than E20, depending on the state, and many motorists are unaware they can ask for it.
Over the past few months, consumers have flooded social media with complaints of engine wear, lower fuel efficiency and reduced performance.
Last week, several motorists joined a protest in Delhi, organised by an entrepreneur who often backs the opposition Congress party in TV debates, accusing the government of unilaterally imposing E20 and leaving them with higher servicing costs and little choice at fuel stations.
Prime Minister Narendra Modi's government has dismissed the complaints as "misleading" and "social media misinformation".
In a statement last month, the government said the fuel was rolled out after extensive testing and does not damage engines. It has also issued statements on the benefits of E20, external and to bust what it called the "more colourful myths that have circulated on social media".
The government has also enlisted experts and automakers to defend E20.
Last weekend, in an unusual show of unity, six automakers joined a government press conference to say that years of testing and service data showed no evidence of widespread vehicle damage from the mandatory 20% ethanol blend.
Rahul Bharti, senior executive officer for corporate affairs at Maruti Suzuki, said that India's largest carmaker had serviced more than 15 million older vehicles which were not E20 compliant but found no fuel-related fault.
The automakers did admit that the use of E20 had led to a 3-3.5% drop in fuel efficiency due to lower energy content in ethanol. Some estimates, however, say the actual drop in efficiency could be higher, between 4-12%, external.
From a broader perspective, the government's biofuel push aims to cut India's oil import bill, support farmers and reduce emissions, as ethanol burns cleaner than petrol and is produced domestically from crops such as sugarcane and maize.
Government data suggests it has largely delivered on those goals.
India imports most of the crude oil it consumes, and the disruption to global oil markets during the Iran conflict has strengthened the case for producing more fuel at home. Other oil-importing Asian countries, including Indonesia and Vietnam, are also accelerating plans to expand ethanol-blended and flex-fuel programmes.
Brazil, the model Indian officials often cite, built its ethanol-ready vehicle fleet gradually over four decades before mandating higher blends. Consumers there can choose between E27 petrol and 100% ethanol, with pump prices often making ethanol the cheaper option.
India compressed a similar transition into just three years, moving from 10% ethanol in 2022 to a mandatory 20% blend by 2025 - five years ahead of schedule. Critics say it skipped the step every other major market took first: giving vehicles time to adapt before raising ethanol levels.
More than 75% of vehicles on Indian streets are not E20 compliant, said Puneet Gupta, director of auto research firm Mobility Global. An analysis, external by Thomson Reuters Foundation last year came to a similar conclusion, finding that only some 20% of new petrol vehicles sold in past 15 years were E20-compliant.
The rapid transition has fuelled concerns, especially among owners of pre-2023 vehicles designed for lower ethanol blends, who fear engine corrosion and reduced performance.
Complaints have mounted in recent months, with protesters at last week's Delhi demonstration describing problems they said emerged after using E20.
The BBC spoke to several car and two-wheeler service centres and mechanics in Mumbai. While some said they had seen no issues so far, others reported cases they believed were linked to the higher ethanol blend.
