Petrol is, at times, very confusing. Each fuel retailing group has its own brand names for the fuels it sells and the petrol itself is available in four different ‘flavours,’ generally known as 91, 95, 98 and E10, plus E85 on the odd occasion. It can be very confusing knowing which one to re-fill your tank with.
The numbers are what matters. Standard unleaded petrol is 91. Premium unleaded is both 95 and 98. The ethanol-blended E10 (a mixture of up to 10 per cent ethanol in petrol) is a substitute for 91 in most cars 2005ish or newer. However, it pays to check your user manual.
Those numbers – 91, 95 and 98 – are the so-called ‘octane rating’ of the fuel. They’re all about the same in terms of the energy in the fuel. What octane really is, is an index of a fuel’s resistance to burning too early inside your engine – if that happens, it causes ‘pinking’ or ‘pinging’ (same thing), which is mechanically destructive at high revs and large throttle openings.
Carmakers design engines for a minimum octane rating. If you open the fuel flap of your car and it says ‘unleaded petrol only’ it means 91 octane fuel is OK. If the fuel flap says ‘premium unleaded only’ it means you need to use at least 95. If the fuel flap tells you to use 98, that’s what you need to do.
It can’t hurt your engine if you use a higher octane fuel. So, for example, if you use 95 or 98 in an engine designed for 91, that’s OK. However, it’s not acceptable to put in a lower octane fuel than the minimum recommended by the manufacturer. Using 91 in an engine designed for 95 or 98 is potentially destructive.
If you run them on a higher octane fuel than the minimum recommended, you will get either better economy or more performance (depending on how you drive). But in practice, the improvement is tiny, and the price premium of the higher octane fuel often eclipses the economy benefit from running it – in other words, it’s not an economically rational choice to run 98 in an engine designed for 91, even though it might run slightly better.
