Can you mix E5 and E10 fuel? What happens if you put new and old grade petrol in the same tank?

Everything you need to know about mixing 95 and 98 RON petrol, including whether it will damage your car, plus how to use a fuel checker for your vehicle

Earlier this year the UK’s standard petrol changed, from E5 to E10.

The numbers relate to the proportion of bioethanol in the fuel and mean that there are now two distinctly different types of petrol on sale in the UK.

Sign up to our NationalWorld Today newsletter

The i newsletter cut through the noise

Most modern cars can run safely on E10, which is now sold nationwide as regular 95 RON unleaded but many filling stations also still sell E5 under as premium or super unleaded with the higher 98 RON octane rating.

That means in many circustances, drivers have a choice of which fuel to use but, as the recent fuel crisis showed, sometimes drivers simply need to fill up with whatever is available, leading to questions around how safe it is to mix the two types of unleaded.

Can I mix E5 and E10 petrol?

Although E5 and E10 have different compositions, the Department for Transport says it is entirely safe to mix the two grades.

Like mixing regular and super unleaded in the days of E5 petrol, mixing E5 and E10 in the same tank will not do any harm to your vehicle.

In its guidance on E10, the DfT says: “If your vehicle is compatible with E10 petrol, there’s no reason you can’t mix the 2 grades of petrol (E5 97+ and E10 95+). It’s perfectly safe to mix them in the same tank or fill up with E5 if E10 is not available.”

What is the difference between E5 and E10?

Both E5 and E10 are unleaded petrol but E10 contains a higher proportion of bio-ethanol - up to 10% compared with a maximum of 5% in E5.

This higher bio-ethanol content - obtained from renewable sources - helps to reduce the CO2 emissions from vehicles but slightly reduces their fuel efficiency.

Why has E10 petrol been introduced?

The Government says the introduction of E10 petrol will help reduce transport-related air pollution.

It estimates that changing to E10 will cut the UK’s CO2 emissions by 750,000 tonnes a year - equivalent to removing 350,000 cars from the road.

Can all cars run on E10 petrol?

The vast majority of modern cars can run safely on E5 or E10 but there are up to 600,000 older models in the UK which aren’t compatible with the newer fuel.

Any car built since 2010 is compatible with E10 fuel and most built since 2000 can also use the fuel without damage. However, some models from between 2000 and 2010 face problems using the fuel, as do hundreds of thousands of older and classic cars.

What if I have to use E10 in an incompatible car?

Some drivers have claimed to experience poor running when filling up with E10 but a single fill-up of E10 is unlikely to cause lasting damage to incompatible cars.

The damage caused by E10 to older cars largely comes about due to the long-term effects of the ethanol on perishable components, so a single emergency fill-up won’t do too much harm.

The DfT advises: “Using a single tank of E10 petrol in a vehicle that is not compatible should not be a major problem. Just make sure you fill up with the correct E5 (‘97+ octane) petrol grade next time.

Unlike putting petrol into a diesel engine, you shouldn’t need to drain the tank. On a one-time basis, your vehicle will not suffer engine damage as a result. Prolonged use of E10 petrol in a non-compatible vehicle, however, may cause harm and is not recommended.”