At a British petrol pump, the expensive nozzle often promises more: higher octane, sometimes a special additive package, and a name that sounds like a reward for a well-loved engine. But the useful question is narrower than “Is premium better?” It is: what fuel was this particular engine designed to use?
There is a naming trap. In UK regulations, ordinary 95 RON unleaded has historically been called “premium”; drivers usually mean the dearer 97 RON or higher grade when they say “premium petrol”. The Department for Transport calls that higher grade “super”.¹ Here, “super” means the 97–99 RON choices commonly set against standard 95 RON.
What the octane number measures
RON stands for Research Octane Number. It is a measure of resistance to unwanted self-ignition in a spark-ignition engine. In normal operation, the spark plug starts a controlled flame. Under high pressure and temperature, part of the mixture can ignite too early or too abruptly, causing knock. An engine designed for high compression or significant turbocharger boost may need a fuel with greater knock resistance.²
A higher RON number does not mean that a litre contains more energy, or that it is automatically cleaner, richer or more powerful. It describes one important property of the fuel. An engine that cannot make use of the extra knock resistance has no magic route to extra power simply because the pump displays a bigger number.
Modern engine controls can complicate the picture. Some cars are calibrated to run best on high-octane petrol but can protect themselves on a lower grade by changing ignition timing or boost. Others are designed for 95 RON and gain little from an upgrade. The handbook or fuel-flap guidance is therefore more useful than a general rule about all cars.
What do the tests show?
The American Automobile Association tested regular and premium petrol in cars designed for regular fuel, using controlled laboratory procedures. It found no significant gain in horsepower, fuel economy or tailpipe emissions from the higher grade in those tested vehicles.³ That is evidence against buying octane for its own sake. The study used US fuel grades and a limited selection of vehicles, so its numbers are not a universal verdict on every British engine or branded additive blend.
The inverse is also important: the study did not test the consequences of using a lower-than-required grade in a car designed for premium.³ If a manufacturer specifies 98 RON, using 95 RON to save money may reduce performance or contradict its instructions. “Recommended” and “required” can mean different things; read the exact wording for the model and engine, not just the badge on the boot.
What about an engine that specifies 95 RON but promises a little more performance with 98? That is a vehicle-specific claim worth checking against the manufacturer's data or a careful comparison. A small difference under heavy load may be real yet barely noticeable on a gentle commute. The purchase decision is then about the price of that difference, not whether the dearer fuel is intrinsically superior.
The second number on the pump
Octane is only half the UK forecourt puzzle. Standard 95 RON petrol is generally labelled E10, meaning it can contain up to ten per cent ethanol. The higher-octane super grade is generally E5, containing no more than five per cent. These are separate properties: E10 and E5 describe ethanol limits, while RON describes knock resistance. The UK government's guidance says stations offering two grades will stock E10 at 95 RON and E5 at 97 RON or more.¹
This distinction matters particularly for an older or specialist vehicle. Most petrol cars can use E10, and the government offers a compatibility checker. Some classics and particular older models may need E5 regardless of whether their engines would otherwise benefit from a higher octane rating.¹ In that case, buying super is about fuel compatibility, not chasing performance.
Ethanol also has a different energy content from petrol. The Department for Transport estimates that switching to E10 can lower fuel economy by around one per cent, usually too little to notice amid normal variation in traffic, tyre pressure and driving style.¹ It does not follow that every tank of E5 super will repay its price premium in extra miles. To test value, a driver would need a repeatable measure of cost per mile, not a single unusually good trip.
The additives question
Branded fuels may differ in detergents or other additives as well as octane. Their marketing can make it hard to isolate which ingredient supposedly improved what. A claim about keeping an engine clean is different from a claim about knock resistance. AAA's comparison deliberately controlled fuel composition to test octane; it should not be misquoted as a definitive test of every additive package on the UK market.³
Nor should a smoother-sounding engine after one fill-up be treated as a controlled experiment. Weather, route, load and expectation can alter what a driver notices. If there is a mechanical problem—persistent knocking, rough running or a warning light—the sensible next step is diagnosis rather than assuming the costliest nozzle is a repair.
So when is super worth it?
Start with the handbook. If it requires a higher grade, buy it. If it recommends one for optimum performance, decide whether that benefit matters in your driving. If it specifies 95 RON, there is little reason to expect extra octane alone to transform it. If the vehicle is not E10-compatible, seek the appropriate E5 fuel even when octane is not the main concern.¹³
The premium at the pump buys a different specification, not a universal upgrade. The clever purchase is the grade that matches the engine and the use you actually have.