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Should drivers pay more for performance fuels?

Motorists assume a more expensive fuel has benefits. That may be true, but probably not for everyone.

As long as anyone can remember, oil companies have been offering different grades of fuel at their petrol stations. The advantages of spending more are supposed to be higher mileage, higher performance and better engine protection but the rationale is unclear. Turning it around, does it mean their cheaper grade provides lower mileage, poorer performance and faster engine wear?

Octane The “research octane number” (RON) is a standard measure of fuel performance, but it has a specific meaning. The higher the RON, the more compression (and temperature) the fuel withstands inside the piston before it ignites. Fuel that ignites prematurely causes shockwaves that can eventually damage an engine (“knocking”), and also reduces its efficiency. Generally, “standard unleaded” has a RON of about 95, and “super unleaded” 98 plus.

Low powered petrol engines compress fuel considerably less than those designed to be high performance - and this is the key to whether higher octane fuel benefits you. Petrol is more likely to ignite early and “knock” the engine when under high compression. In cooler engines that compress less, knocking is less likely to occur in the first place so higher octanes can’t improve them.

Additives Some gas companies make additional claims for their performance fuels. For example, Esso states that their higher octane premium grade petrol has double the detergent additives to give better engine performance by giving a deeper clean. So do premium priced fuels offer improvements for all drivers, irrespective of their engine model?

Detergent additives make up a small fraction of the production cost of a gas station fuel, so it is hard to understand why you would put less than the optimum amount of detergent in any grade of fuel. However, more detergent makes sense in an engine prone to knocking, because it can hamper fuel combustion and produce more unburned contaminants. So once again, these benefits probably apply to the higher compression engines more likely to need them.

Mileage and performance

Since knocking impairs combustion efficiency, improved mileage, speed and acceleration should be achievable if you use a grade of fuel that controls it better. The problem is telling if your engine needs any assistance. The only way to assess this for most drivers will be to fill up on an empty tank and compare the miles achieved over similar routes.

On a race track, small improvements in acceleration and speed really matter. However, the performance bottleneck for a performance car on the open road is really down to skilful use of the brakes, gears and steering wheel. Fuel will contribute much less to performance than your foot.

The ideal fuel is the one that suits your car. Quirks of any engine design can have significant effects and so will your driving style. To get the most from your pocket, we recommend you experiment with a full tank of each. As soon as you change your car, the answer could change too.