If you've filled up at a UK forecourt any time in the last few years, you'll have noticed the pump handles marked "E10" and "E5". But plenty of drivers still aren't entirely sure what those labels actually mean, or why it matters which one they choose.
The short version: it's all about how much ethanol is blended into the fuel, and for most modern cars, it makes little practical difference. But for a meaningful minority of vehicles on UK roads, picking the wrong one over time really can cause problems.
Here's everything you need to know about E5 and E10 petrol, which one your car should be running on, and what happens if you get it wrong.
What is E10 Petrol?
E10 is standard unleaded petrol blended with up to 10% bioethanol, a renewable fuel produced from crops such as wheat and sugar beet. Since 1 September 2021, E10 has been the default 95-octane "regular" unleaded sold across England, Scotland and Wales, with Northern Ireland following in November 2022.
The switch was a deliberate environmental measure. Because bioethanol is a renewable component rather than a fossil fuel, blending more of it into standard petrol lowers the overall carbon footprint of every tank sold. The change is estimated to cut UK transport CO2 emissions by roughly 750,000 tonnes a year. That’s comparable to removing around 350,000 cars from the road.