Superfuels: are they worth the money? - The results

The results (scroll down to see our table below) are repeatable to within 1% - i.e. plus or minus 0.5%, so if we did the tests again, we'd expect almost identical figures.

They suggest high-octane fuels deliver little improvement in fuel economy. The majority of ordinary 95 octane fuels performed just as well as the superfuels, which cost you up to 9p a litre more.

The top performer for average fuel consumption was Sainsbury's 97 premium fuel, but it delivered just 0.5mpg more than Sainsbury's - that's six more miles on a tankful of fuel.

BP 97 Ultimate delivered only 0.1mpg more than BP 95 in the short-term test, while Esso's premium fuel delivered just 0.1mpg more than its 95 fuel.

In the longer-term test, we saw no mpg gains. Similarly, there was little improvement in performance when we switched over from using regular fuels to running on superfuels.

Finally, on the Golf GTI, there was little difference between the fuels in performance or fuel economy.

However, the VW Group still advises that its cars with FSI and TSI engines are run on 98 octane fuel, because it's 'more efficient'.

Of course, we're not able to assess the fuel companies' claims that these products prolong engine life, and de Nayer argues that after this test, second-hand buyers will probably be the real beneficiaries.

'If you use fuels that potentially clean your engine then you might reap the rewards, because a 40,000-mile engine that's been coked up will need cleaning,' explains de Nayer.

'That's little use to someone with a brand new car, though. I know it made me blanch when I went for my first fill with BP Ultimate and paid 106p a litre, having paid 93p for a litre of 95 octane at Asda - you really have to be devoted to the idea of cleaning your engine internals to be prepared to do that every time you fill up. It's like having a hole in your pocket.'

So what do you save if you stick to 95 octane fuel? According to official figures, a 1.6-litre Ford Focus should do 42.8mpg.

So, if you do 12,000 miles a year, you'll spend £1226 on regular fuel at the current national average of 96p a litre.

If you use a premium fuel, which costs an average 103p a litre, your annual fuel bill will be £1309 - a difference of £83.