In a world where fast food restaurants want diners to super-size, airlines would prefer you to book business class, and car dealers try to sting you for extras, it’s not surprising fuel companies offer an ‘upsell’ to drivers: premium fuel.
Posh versions of unleaded and diesel fuel are available from most branded and supermarket fuel suppliers. They are claimed to give better fuel economy and keep engines running in optimal condition.
Some car makers demand that their petrol engines are run on such fuels – often called super unleaded. However, these are more expensive. And proving their worth if a car doesn’t specifically have to be run on premium fuels is almost impossible.
To help drivers decide whether they should use premium fuels, we compare costs for the typical car and driver, and investigate the claims made by fuel companies.
What is premium fuel?
There are two things that differentiate premium fuels from the regular stuff. The first is the official rating of the fuel. For unleaded, this is known as the octane rating. For diesel, it’s the cetane rating.
The octane rating shows how well the fuel resists pressure and premature ignition as it is compressed by the piston. Cetane is a measure of how quickly diesel fuel ignites under pressure.
The other area of difference are the additives in the fuel. These are claimed to reduce friction and keep some of an engine’s most precious components clean. The result is supposedly more efficient performance.
Which cars need premium fuel?
Petrol engines with a high compression ratio – as a rule of thumb, that’s generally engines fitted to high-performance vehicles – often require super unleaded fuel.
However, no car maker stipulates that diesel engines are run on premium diesel fuels. If you own a diesel car, deciding whether to fill it with the posh stuff boils down to whether you believe the fuel supplier’s claims about improved economy (miles per gallon) and superior engine cleaning power.
How to check if a petrol car needs premium fuel
This is the easy bit. Look inside the fuel filler flap. There should be sticker that names the type of fuel (unleaded) and the minimum required octane (RON) rating. Regular petrol is around 95 RON, super unleaded varies between 97 and 100 RON.
If there isn’t a sticker, read the vehicle handbook, or speak with the car maker’s customer service department.
How much more expensive is premium petrol?
According to the website petrolprices.com (as of 12 September, 2017), the average price of a litre of unleaded petrol is £1.19 in the UK. Super unleaded is £1.29. The difference means that a driver of a new Volkswagen Golf 1.4 TSI would pay an extra £5 for each tankful.
That may not sound like a lot. But over 10,000 miles of driving it adds up to an extra £86. Do you really want to spend the extra when there’s no independent evidence to suggest super unleaded fuels are better for petrol engines?
How much more expensive is premium diesel?
Petrolprices.com gives an average UK price for a litre of diesel of £1.20. Premium diesel is £1.33. In a Ford Focus 1.5 TDCi diesel – one of the UK’s best-selling family hatchbacks – filling its 53-litre tank with normal diesel is nearly £6.90 cheaper than the fancy fuel. Over 10,000 miles of driving, that is a £70 saving, enough for another tank of fuel.
Will I get better fuel economy with premium fuel?
Fuel manufacturers claim an improvement in economy. However, there is no way for everyday drivers to measure this with any certainty. You will never be able to exactly replicate the driving conditions when using up a tankful of regular fuel followed by a tankful of premium fuel.
What about the cleaning benefit?
All fuels need to contain cleaning agents to be certified for European use. If they didn’t, engines would clog up with mucky deposits. Use a reputable brand of fuel and your engine should stay clean over time and there shouldn’t be the need to fill with premium fuel.
However, some drivers and mechanics say old engines with an unknown history can benefit from the additional cleaning power of premium fuels, made by companies such as BP, Esso and Shell. In this case, using a couple of tankfuls of super unleaded or premium diesel should remove any harmful deposits. The driver can then switch back to a good quality, regular fuel.
James is a motoring journalist and former magazine editor at BBC Top Gear and Auto Express. He has scooped, reported on and reviewed most new cars of the past 20 years, and currently contributes to the Driving section of The Sunday Times.