E10, E5 Super-Unleaded and the Real MPG Penalty, Which Petrol Is Actually Worth Paying For?

E10 has been the UK's standard unleaded grade since September 2021, but the questions haven't gone away. Does it cost you more in MPG? Does it damage older engines? Is paying 12p extra per litre for E5 super-unleaded ever worth it? The answers are more nuanced than either side of the debate suggests.

What E10 actually is

E10 contains up to 10% bioethanol blended with 90% petrol. The previous standard, E5, capped ethanol at 5%. The switch was a climate measure, the Department for Transport estimates it cuts CO₂ by around 750,000 tonnes a year, equivalent to taking 350,000 cars off the road. The trade-off is in the chemistry.

The MPG penalty is real but small

Ethanol contains roughly two-thirds the energy of petrol per litre. So in pure thermodynamic terms, E10 should cut fuel economy by around 1.5%. Real-world testing by What Car? and Honest John consistently lands at 1-3%, meaningful over a year, but rarely enough to justify paying 12p more per litre for E5 super-unleaded. On a typical 12,000-mile-a-year car doing 45 mpg, the MPG hit costs around £45 a year. Switching to E5 super costs around £200 more.

Which cars genuinely shouldn't use E10

The gov.uk compatibility checker is the definitive source, but the rough guide is: any petrol car built before 2002 should avoid E10, and many built between 2002 and 2011 should be checked individually. The risk isn't immediate engine damage, it's ethanol's tendency to absorb water and corrode older fuel system components, particularly rubber seals, brass fittings and uncoated steel tanks. Classic cars, older motorcycles and small two-stroke engines (lawnmowers, chainsaws, outboards) are the highest risk.

When E5 super-unleaded does make sense

For most modern cars, paying for E5 (typically branded Shell V-Power, BP Ultimate, Esso Synergy Supreme+) buys you a small octane and detergent benefit, not a fuel-economy benefit. It's worth it for high-performance turbocharged engines that specify 97+ RON, and for cars that sit unused for months, ethanol absorbs atmospheric water, and a tank of E5 will store cleanly for far longer than E10.

The forecourt-to-forecourt gap dwarfs the grade gap

The price difference between cheapest and most expensive E10 within five miles of any UK postcode regularly exceeds 15p per litre. That's around £100 a year for the average driver, far more than any E10-vs-E5 calculation. As we covered in why you're probably overpaying for petrol, the biggest wins come from picking the right forecourt, not the right grade.

E10 is rarely the wrong choice for a modern car. The conversation worth having isn't E10 vs E5, it's whether you're paying a 15p premium per litre because the nearest supermarket is two miles further than the petrol station you always use.

References: gov.uk E10 compatibility checker · Department for Transport CO₂ projections · What Car? real-world MPG testing · CMA fuel price transparency scheme