Fuel Types

We need fuel to keep our cars and other vehicles working on the road. One day, petrol and diesel will run out so we will need to find alternative fuel resources. However, for the mean time most cars need petrol and diesel fuel to work. Here you can learn a bit more about fuel and the different types available.

Types of Fuel and Octane Ratings

Petrol’s octane rating is a measurement of the fuel’s ability to resist engine knocking. Knock occurs when the fuel-air mix in the cylinder explodes instead of burning in a controlled way. This shockwave moves within the combustion chamber, and creates a metallic ‘pinging’ sound.

An octane rating is often referred to as an ‘anti-knock index’. If fuel has a high octane number, it will have a higher resistance to engine knocking.

Octane Numbers

Usually, there are three different octane numbers associated with all petrols. Petrol’s Research Octane Number (RON) is measured under simple test conditions. Petrol’s Motor Octane Number (MON) is measured under tougher test conditions and at higher engine speed and temperature.

The average of these two values is what becomes related closer to actual driving conditions. This value is known as the Road Octane Number, and is what should be used in filling stations.

Occasionally, some filling stations will confuse these different octane numbers in a bid to embellish on their octane rating claims, and advertise their fuel’s Research Octane Number, which is higher than the Road Octane Number. In many European countries, the Research Octane Number is advertised on pumps, so a much higher octane value is common when travelling in certain countries.

Unleaded (95 RON)

The most common petroleum on British forecourts.

Diesel (Heavy oil)

Diesel automatically ignites and burns when it is compressed to a very high pressure. The released energy is contained by the engine and powers the vehicle. The key difference between diesel and petrol engines is auto-ignition. A spark plug ignites the fuel in a petrol engine whereas a diesel engine auto-ignites. Diesel has a Cetane Index and Number rather than the Research Octane Number (RON) that petrol has.

Super unleaded (98 RON)

Using Super unleaded in vehicles that require a higher octane rated fuel is an additional expense to owning a powerful vehicle. There is more information on super unleaded and premium fuels at the bottom of this guide.


Bio-diesel is a more environmentally friendly fuel slowly becoming available across the UK. It is produced from renewable energy sources such as sugar beet, rape seed and sunflowers and is a biological substitute for regular diesel. Bio-diesel vehicles are more environmentally friendly than conventional cars which run on petrol and diesel because the fuel is not as toxic and does not produce as many damaging exhaust emissions.

Leaded (Four star)

Leaded Four Star petrol was removed from sale on British forecourts on 1st January 2000. However, Leaded Four Star is now sold in a small amount of licensed stations in the UK.

Petrol vs. Diesel

Both petrol and diesel have advantages over one another which, depending on what factors are most important to you, will influence your choice in fuel. Check out the table below for a quick look at which would be the preferable fuel for each category.

Car Tax

Some diesel vehicles are much more efficient than their petrol equivalent and they expel fewer co2 emissions (this does not mean they are less harmful to humans) which results in paying much less tax. In fact, a test by Which? showed that a 1.4L Diesel Volkswagen Polo BlueMotion is exempt from vehicle tax charges under current rules. However, the equivalent petroleum edition is hit with an annual bill of £120.

This guide does not take into consideration the latest diesel tax charges in major cities.

Servicing Costs

Routine maintenance and repairs are also a factor that can vary between petrol and diesel vehicles with diesel seen to be the more expensive. However, the fact is that a diesel car would need to be serviced less compared to a petrol car, therefore levelling out the amount it would cost you if you plan on keeping your vehicle for a 3-4 year period.

Resale Value
Unless you’re driving a classic sports car, your vehicle depreciation is likely to be one of the biggest expenses you’ll be faced with when it’s time to sell. Diesel cars were generally in high demand, thanks to their fuel economy and little-to-no tax rate. However, with the latest changes from the government, petroleum SUVs are one of the most popular vehicles to buy now.

Although it’s well known that a petrol engined car offers better performance, thanks to the advantages in diesel engine technology, the diesel engine is closing the difference between speed, noise and ride.

If you are choosing between either the petrol and diesel versions of the same car then a test drive should point out any differences between performance for you.

Road noise
The common perception of the clattering diesel engine, although still prevalent when compared to a petrol engine, is becoming much less of an issue as technology progresses.

If you often use your car for towing a trailer or caravan then the increased torque offered by a diesel engine allowing you to pull more may play an important factor in your choice of engine type.

The effect your vehicle has on the environment is a hot issue amongst many motorists. Both petrol and diesel produce emissions that have negative affects on our environment however diesel cars achieve better fuel efficiency and therefore produce less emission over a given distance and compared to petrol.

Price difference
The price of diesel per litre can be higher than that of petrol in the UK, however the increased fuel efficiency of a diesel car will mean you have to fill up less making for less of a difference between the two.

If price is your main concern then it’s important to consider all the costs associated with the each fuel type over the life of the car as opposed to just the current price at the pumps.

Which is the best?

When comparing diesel and petrol prices, it is important to remember one thing. One of the key attractions of a diesel powered vehicle is that its engine is generally much more efficient than a petrol run equivalent. This is why diesel cars are usually slightly more expensive.

Clearly, diesel is more economical in the long run even if it is more expensive per litre. This is slowly changing though. The newest ULP engines are squeezing out more mpg for the same amount of petrol. If petrol cars continue to improve and match the efficiency of their diesel counterparts, then car companies could well find themselves having to think of new reasons why people need to buy diesel.

There is no way to give a definitive answer on whether petrol or diesel is the better choice of fuel given that there are so many variables to consider that will appeal to different needs and driving habits. For example although diesel will give you more miles to the gallon in laboratory condition, driving a petrol car efficiently can give you better performance than if you were to drive a diesel inefficiently.

It’s best then to evaluate your current driving habits and consider exactly what you value in a car when deciding on either a petrol or diesel engine.

Super unleaded and premium diesel

Many of the large petrol companies have launched ‘super fuels’ – petrols and diesels that have a higher research octane level. These fuels are said to increase power in many vehicles, deliver less pollution than regular fuels and help to maintain a cleaner engine. Amongst these ‘super fuels’ are Tesco’s Super Unleaded 99 Octane petrol, supplied by Greenergy, now sold at many stations across the UK.

One of our users commented that using higher octane fuel than your engine requires actually gives no benefit and may be a waste of money. This is because virtually NO engines require 98 RON over 95, and the market for ‘super’ fuels seems to be based on people’s misunderstanding of octane ratings and the placebo effect of filling up with ‘more powerful’ fuel – making motorists think their engine is running better in some way.

What do the fuel companies say then, to justify the “increased power” claims for the super grade fuels? Some companies say that while all fuels contain cleaning additives, ‘super’ fuels contain more or better detergents to keep the injectors cleaner than standard fuel. Others say the fuel is a few percent denser which gives slightly more power per litre. These benefits may be marginal though in comparison to the extra cost involved so it is worth ensuring that your engine will actually benefit before filling up.

Some engines actually do need higher octane fuel, such as race engines with very high compression, and some turbocharged engines, such as the import version of the Nissan Skyline. Also, a few vehicles, such as the new BMW K1200R motorbike, can sense knock and adjust their engine tuning to take advantage of higher grade fuels. Another user commented that the 2004 BMW 330 also does this, according to the driver’s handbook it makes 231 BHP on 98 octane and 221 BHP on 95. This ability is apparently widespread amongst German performance cars using Bosch / Siemens electronic engine controls.

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