Petrol 95 or 98: which does your car really need?

The 95 and 98 are octane ratings. Octane measures one thing only: how well the fuel resists igniting under pressure before the spark plug fires. That premature ignition, called knock, is what high-performance engines have to avoid. A higher number means more resistance to knock, nothing more.

It does not mean more energy, more power or a cleaner burn in itself. Those ideas get attached to 98 by marketing, but the octane number is purely about knock resistance. Cleanliness comes from additives, which are a separate thing a brand may add to any grade.

Every engine has a fuel it was built around, and the maker states it, usually on the inside of the fuel flap and in the manual. Most ordinary cars are designed for 95. Some higher-compression, turbocharged or performance engines require or recommend 98, and a few will say so plainly with the word "minimum".

If it says 95, your car is tuned for 95 and gains essentially nothing from 98 in normal driving. If it says 98 required, use 98: that is the fuel the engine was mapped for, and going lower can trigger the knock protection and cost you performance and economy.

Worth it if the car requires or recommends 98: you protect the engine and get the performance and efficiency it was designed for. On some engines that merely recommend it, 98 can also help a little when working hard, towing or in the heat.

Wasted in a car designed for 95 and driven normally. You will not feel more power or see better economy worth the price, you will just pay a few cents more a litre for a property your engine never calls on. The occasional tank does no harm, but it is not an upgrade.

Once you know the grade your car needs, the money is in where you buy it. The price of 98 at a dear station can dwarf the small octane premium, and the gap between two nearby stations for the same grade is routinely 15 to 20 cents a litre. Pay for the octane your engine needs, then let a quick comparison find the cheapest pump for it.

Fillz shows the grades and prices around you side by side, so you never overpay for 98 you do not need, or drive past cheaper 98 you do.

Will 98 give my 95 car more power or better mpg?

In normal driving, no meaningful amount. The engine is tuned for 95 and cannot use the extra knock resistance, so you mostly just pay more.

Is higher octane cleaner fuel?

Octane and cleanliness are separate. Detergent additives keep the engine clean, and a brand can add them to 95 or 98 alike. The octane number itself says nothing about cleaning.

Can I use 95 if the car asks for 98?

If it says recommended, occasional 95 is usually fine, with a small loss of performance. If it says required or minimum 98, stick to 98 to avoid knock and protect the engine.

Does 98 damage a normal engine?

No. Running 98 in a 95 car is harmless, just unnecessary. The only thing it hurts is your wallet.