Some manufacturers specify that you should run your car on 95 or 98 octane minimum rather than on regular ULP (unleaded petrol). Some of these high-performance fuels make claims that they will clean your engine, help protect it and even give you better fuel economy. But these fuels also cost more per litre, so is it worth using them if your car doesn’t need to run on 95 or 98 octane?
Octane rating is the ability of fuel to resist engine knock at high compression ratios. Engine knock is when the spark doesn’t ignite the fuel at the right time in the engine, causing a metallic pinging sound, and this increases the compression ratio in the piston. In general, it’s not good – you put the engine under more stress, but get less power for it.
So, all nerdy engine facts aside, will you notice any difference when driving?
If you have a very standard car (e.g. Honda Civic, Toyota Corolla) and you drive sensibly and carefully, then it’s highly unlikely you will notice any difference putting premium fuel in your tank. Likewise, if you have a high performance car and you put low octane fuel in, but you’re easy on the gas and don’t try to win any races, it’s unlikely you will notice any difference.
But with high performance sports cars some experts say that high octane fuel improves performance and fuel economy. In my own experience, I owned a Subaru WRX STI back in the day – it was a 1998 Type R, version 4 with about 300bhp after some very minor fettling. That car definitely ran better on 98 octane fuel than regular unleaded. Whereas my current car runs on regular unleaded and I notice no difference if I put premium fuel in it.
Putting low octane fuel in the WRX resulted in ‘pinging’ under hard acceleration – the fuel didn’t have the ability to resist engine knock under those high performance requirements. When it’s the other way around, i.e. putting higher octane fuel in than is required, you might notice a bit of extra power, but it’s likely to be negligible. You certainly won’t be doing your engine any harm by having the premium fuel in there.
Some manufacturers recommend a specific brand and type of fuel. We’re not sure whether that’s more of a marketing ploy than strict scientific recommendation.
What about the occasional tank of premium fuel?
The occasional tank of premium fuel in your older engine won’t do any harm at all. Some premium fuels do help clean the engine and you may find that it will improve your fuel economy slightly. However, as fuel economy is affected by so many factors such tyre pressure, accelerator use, length of trips, types of trips, wet roads, strong winds, the passengers you carry, your use of air conditioning and more, it’s very difficult to assess whether it would make any difference unless you keep records of your average fuel economy for several months, and then switch to premium petrol for several months to test that. In some cases, the simple act of switching fuels in order to see if your car is more economical will subconsciously cause you to drive more economically.