The introduction of a ‘greener’ petrol to meet EU regulations could cost UK drivers billions of pounds a year and increase harmful CO2 tailpipe emissions.

The move to introduce E10 fuel, which is expected to happen this year, has been branded as "irresponsible" by consumer  magazine What Car? after it undertook the first ever real-world tests on the new blend of petrol. Until now, the fuel had only been tested in laboratory conditions and the potential impact on fuel economy had not been communicated to motorists.

The E10 fuel contains 10% bio-ethanol and is being rolled out across the UK as part of the Government’s commitment to reducing greenhouse gas emissions and conforming to the EU’s Renewable Energy Directive. This requires 10% of road transport energy to be from renewable sources by 2020.

However, What Car?’s testers discovered that E10 is less efficient than the current E5 (up to 5% bio-ethanol) blend of fuel across every engine type tested. This means cars have to use more of the new fuel, costing drivers much more each year.

Editor-in-chief Chas Hallett is calling for the Government to carry out comprehensive, UK-focused testing in order to better understand the financial impact of the new petrol.

"The US Environmental Protection Agency estimates that the detrimental effect of E10 on fuel economy is between three and four percent, but even our small sample of tests proves otherwise," he said.

"To lead consumers into E10 without fully communicating the significant impact on fuel economy, particularly for drivers least able to absorb the extra costs, is irresponsible."

What Car? tested E10 against E0 ‘pure’ petrol so we could directly compare our results with the US EPA’s. The cars used were a three-cylinder turbo (Dacia Sandero), a naturally aspirated car (Hyundai i30), a hybrid (Toyota Prius+) and a four-cylinder turbo (Mini Paceman).

The Sandero struggled the most, returning an 11.5% drop in economy. The 99bhp i30 was almost as bad, managing 9.8% less miles on E10.

It’s not just economy that is harmed by the use of E10 – CO2 tailpipe emissions also increased in every vehicle tested by What Car?, although the Low Carbon Vehicle Partnership asserts that these increases would be partially offset by the renewable properties of bio-ethanol and the fact that the crops used to produce it absorb CO2 while growing.

Overall, the What Car? tests suggest that more powerful cars cope better with a higher ethanol content, leaving smaller cars – often bought by those on a tighter budget – worst affected.

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