My Auto Shop

Look, a petrol car needs petrol and a diesel car needs diesel. These are things we know. If you don’t, well it’s written on your fuel cap or on your Registration.

Then how come there are 2, sometimes 3 different kinds of petrol and only 1 kind of diesel? Why are there numbers on petrol? Who knows?

We do and in about 2 minutes you will too.

Do I use Petrol or Diesel?

Your fuel cap should tell you which fuel to use. If it doesn’t but it does say “UNLEADED FUEL ONLY” then your car takes petrol.

If that fails, check your registration papers, it should say on the slip whether it’s petrol or diesel.

If you’re really struggling, if there’s another card above the registration with a barcode and a km’s indicator then it’s diesel. That’s your diesels “Road User Charges”. If you only have the registration card in the corner of the windscreen and no second slip, then petrol it is.

What’s the difference between Petrol and Diesel?

Petrol is more flammable, lighter, and is made in a different way than diesel. Petrol comes in different levels of octane. They both come from crude oil. However there are other ways to obtain diesel, such as biodiesel, which comes from plants.

Diesel has more energy than petrol, which is why you often hear people getting better mileage in diesels. you need less diesel to get the same power as petrol. Diesels also don’t have spark plugs and create combustion from compression.

A lot of that is super complicated, but here’s the main thing:


So what do the numbers on petrol like “91”, “95”, and “98” mean?

Those numbers are the octane rating of the fuel. Basically, the higher the number, the cleaner, and stronger the explosion. Generally, a car with a higher compression ratio or if it’s turbocharged will require a higher octane. Your manufacturer will generally recommend an octane rating for an engine.

You’re effectively paying for a more efficient and powerful petrol when you buy premium.

For reference, diesel is manufactured to be around 25-40 octane. Which is why you can’t mix the two fuels.

If you have any fuel system or engine issues you can consult our handy articles here. Hopefully, you learned something and if you’d like to learn more, check out our “How To” series here