Should you use premium unleaded in your car?

should you fill your car with premium unleaded

It’s billed as some sort of magical fuel that’ll give you more power and better economy, but should you use premium unleaded in your car?

This is one of those ones that’ll start a debate quicker than just about anything else on the planet. A stretch? Maybe. But a lot of people have strong opinions about whether you should be pumping 98RON premium unleaded in your car if the car maker recommends something like 95RON.

Not sure about you, but I’ve seen plenty of roadside billboards and Internet ads telling me that pumping the good stuff into my car will be like a “fitness program for your engine”. And the thing is, that’s kind of true. And there endeth the debate. Nope, not even close.

Now, don’t get me wrong, premium unleaded (98RON) is the best fuel you can get for an engine that’s been tuned to run on it. It’ll give you all the power the maker claims your car makes and give you the best possible fuel economy, it’ll burn cleaner too. But, if your engine has been tuned to run on 95RON then running it on 98RON will only deliver a marginal improvement. And the improvement will be so marginal that no-one on the planet will be able to tell the difference. And those who say they can are just kidding themselves. So, let’s get into this because we’re all busy people.

Let’s start with RON. It stands for Research Octane Number, but don’t get confused, rather than octane, per se, RON has more to do with a fuel’s ability to withstand combustion, or its resistance to knocking (burning too early in your cylinder, or knocking). And if this occurs well it can be destructive…Knock is, in its most basic form, an uncontrolled explosion, or burn in the engine’s combustion chamber, which is designed to handle controlled explosions (or burning) only. Basically, knock occurs when the compression of fuel and air alone ignites in the combustion chamber (without the spark plug) usually in a far corner of the cylinder, while elsewhere in the cylinder the spark plug ignites the mixture as normal – the knock is the two explosions, or burns colliding.

Car makers design their engines to run on a minimum octane rating. Usually, when you open the fuel filler flap, it’ll say whether your car needs ‘premium unleaded’ which means 95RON. Or, if it says you need 98RON then you absolutely must use that. If it says that E10 isn’t allowed, then don’t think a tankful won’t hurt, because it will. Indeed, running the ‘wrong fuel’, say, 95RON in a vehicle designed for 98RON, can lead to some very expensive engine damage.

Most cars built after 1996 carry a knock sensor which is a clever little device that listens for the pre-ignition, uncontrolled explosion, knock or whatever you want to call it and then automatically advance or retard the ignition timing as needed to get to get back to the tipping point between controlled ignition and pre-ignition (knocking). And it’s constantly ‘sensing’ for knock, like 50 times a second.

So that’s why if your vehicle says it needs 98RON you should only ever use that, but what about running 98RON in a vehicle that’s designed to run on 95RON. Will it be a little bit better? Will you get the promised performance and fuel economy boost the marketing bods promise?

Well, no, of course not. Not that you’d notice anyway. If your engine is designed to run happily on 95RON and you tip in a tankful of 98RON then the knock sensor will allow for the engine’s computer to advance the timing ever so slightly to compensate for the denser (more energy rich) fuel. And by advancing the timing, well, yes, you’re getting more power but it’ll be so minute (maybe around 3% improvement) that you’ll honestly never notice. Another benefit will be that because you’re getting that, let’s call it 3%, performance boost you’ll be able to back off the throttle by about 3% and so you’ll likely see a 3% improvement in fuel economy.

So, hang on, if my car is designed to run on 95RON and I run it on 98RON then I really will get an improvement. Well, yes, okay, but it’ll be so small that you really won’t notice. And that boost, fuel economy improvement will be blown away by the difference in pricing between 95RON and 98RON. So, the moral to this story is that you should run the fuel your car is designed to run on, but if you’ve got deep pockets and you want to treat your car to a richer brew then go for it. But just don’t take to the Internet and tell the world that your car now feels like it’s turbocharged or some such nonsense.

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Isaac Bober has been writing about cars and 4x4s for more than 20 years, has worked on some of the country's biggest motoring magazines (remember what they were?), and launched Practical Motoring. Now he's back, back again... to share dad jokes and much more.