Oil prices have fallen by 31 per cent since they peaked at $147 a barrel in July, yet petrol prices have declined by just 5 per cent, despite a threatened supermarket price war.

Motorists will justifiably ask why petrol hasn't fallen further. If petrol had declined as much as oil, we would be looking at pump prices of about 84p a litre, a 31 per cent fall from July averages of 119p a litre.

The usual explanation is that a drop in the oil price takes around four to six weeks to feed through to the petrol pumps. However, this summer, not even a time lag can explain why petrol prices are still so high.

Motoring organisations, including the RAC and AA, have been quick to complain, saying that the oil giants are all too quick to put up the cost of petrol when oil rises but drag their feet when the situation reverses.

However, the high proportion of tax that we pay on fuel prices in the UK, is one reason why pump prices will not fall so far.

At present, more than 50 per cent of the petrol price goes straight to the Exchequer. In other words, half of the cost of filling your tank is a fixed cost that will not fall, no matter how far global crude prices plunge.

"Countries where there is less tax on fuel, experience greater volatility in prices. That is why US drivers have been screaming so much," a spokesman for the AA said.

Supermarkets such as Asda, Tesco and Morrisons have slashed prices significantly, forcing neighbouring oil company sites to follow suit. But prices have remained high in areas where there is no supermarket outlet, says the AA.

The crude oil price may have bounced back above the $100-a-barrel level today, largely due to the surprise cut in production by the Opec oil producers.

But, in the next few months, most market experts expert that the pressure on the oil price will remain as the world's biggest economies slow and there are fears their problems will spread to Asia and stem its voracious demand.

Expect further falls in the price of oil, but don't expect petrol to come tumbling by anywhere near the same margin.

In the meantime, take advantage of the cheapest prices and offers. Just this weekend, prices between central London and central Manchester differed by 5p a litre, with Manchester forecourts selling petrol for 109p a litre.

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