Full guide
Start with what the vehicle requires
The handbook and filler flap come before any general advice. Some performance petrol engines require or recommend a higher octane grade. Older petrol vehicles that are not compatible with E10 may need Super unleaded (E5). Most vehicles approved for standard fuel do not need a premium grade simply because it is available.
For diesel, premium diesel is normally the same public fuel family with an enhanced additive proposition rather than a separate compatibility requirement. Manufacturer guidance still wins, particularly for unusual engines, modified vehicles or equipment.
Work out the benefit the premium would need to deliver
Suppose premium costs 10p/l more. On a 55-litre fill, the extra cost is £5.50. To repay that through economy alone, the car would need to use meaningfully less fuel over the same sort of driving. A tiny change that sits inside normal variation is not enough.
Record distance and litres across several tanks, and compare similar routes, weather and driving. The dashboard economy figure is useful for a pattern, but litres bought and miles travelled give a stronger calculation. Stop the test if the vehicle guidance does not approve the fuel.
When Super unleaded can make sense
Super unleaded may be the correct choice for an engine designed for higher octane fuel, or for an older vehicle that cannot use Petrol (E10). In a car that merely permits it, any performance or economy difference depends on the engine management and driving conditions.
Do not treat E5 and E10 as interchangeable labels for quality. They describe ethanol content, while octane and additive claims are separate characteristics. Check the pump label and the vehicle requirement.
When Premium diesel can make sense
Premium diesel products commonly emphasise detergency and combustion quality. A driver may value that proposition, especially with sustained high mileage, but the result varies by engine and product. Standard Diesel (B7) already has to be suitable for vehicles approved to use it.
The honest answer is therefore conditional. Use premium when required, when a documented product feature matters to you, or when your own repeatable test supports the extra cost. Otherwise, comparing standard diesel prices is usually the clearer saving.
Sources and further reading
Primary evidence for the important factual claims in this guide. External links open in a new tab.