Motorists pulling up to the petrol pump face a choice they rarely entertain. Use the loose change in the glovebox to shell out a bit extra for premium, or stick with the trusty unleaded?

While the range of fuels is limited to the engine type – whether it’s gas, diesel or unleaded petrol – the quality of each fuel impacts engine performance, pulling power, distance or economy. And that is enough to give petrol a second thought.

Petrol or diesel?

First to the diesel.

For that extra torque or pulling power, diesel wins the day. While petrol thrives on revolutions, a tank of diesel is likely to provide more grunt when it comes to getting the vehicle moving, especially when it’s lugging a bit of extra weight.

Premium or unleaded?

While most cars are powered by petrol, drivers face the choice about whether to fill up with unleaded or its premium variety.

The key difference between premium and regular unleaded – both of which are products of crude oil – is the fuel’s ability to resist ‘knock’, which is called the octane rating. And ultimately, adherents of premium say it saves money in the long run by providing optimal engine performance. But how?

In Australia, regular unleaded fuel has an octane rating of 91, while premium is often over 95 and sometimes 98 – keeping engines cleaner and producing less pollution. The octane rating means the fuel reduces the knock when the combustion engine fires. Knock lowers performance and damages engines over the long term. But, according to some, that is chump change for an excuse. They say that better engine designs over time mean the effects of knock are constantly being minimised by manufacturers.

Yes, but what about emissions?

Unfortunately, Australia is in a club of a handful of countries criticised for lagging behind international standards in reducing emissions. By July 2018, all new light vehicles will have to meet the Euro 6 standards which sets an emissions target of 10 parts per million (ppm) of sulphur. Currently, unleaded 91 emits 150ppm of sulphur, while the 95 fuel is capped at 50ppm.

If, as is being touted, the government decides to phase out regular unleaded to pursue the Euro 6 standards, that could drive up the cost of fuel and prices across the economy – given that fuel is fundamental to the delivery of goods.

So in the perennial debate about fuel efficiency, the Australian government looks set to have the final say. Concerns about emissions are likely to trump efficiency arguments as the quality of the air we breathe starts to become a bigger political issue.