Record prices at the pumps could succeed where 6,000 cameras and millions of pounds in road-saftey advertising have failed for decades – by securing compliance with the speed limit.

Driving more slowly will save drivers up to £500 a year in fuel costs, according to a study, which reveals that the most efficient speed is much lower than most people think.

With the average price of petrol at £1.17 a litre and diesel at £1.30 – 20 per cent higher than a year ago – the financial incentive to obey the speed limit has never been greater.

Car manufacturers suggest that the optimum speed for fuel efficiency is between 50mph and 60mph and a recent survey found that two thirds of drivers believe this to be the case. But the study, commissioned by What Car? magazine and based on five cars of different sizes ranging from a 1 litre Toyota Aygo to a 2.2 litre Land Rover Freelander, found that the most efficient speed was below 40mph for all five and as low as 20mph for two.

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Above 40mph, fuel consumption increased sharply and by 90mph the miles per gallon had halved on average.

The study comes as the Government prepares to put in place emergency measures to prevent a strike by Shell oil tanker drivers from creating fuel shortages across the country. Downing Street urged drivers yesterday not to panic buy, which would cause shortages even if fuel deliveries continued as normal.

The study, by Peter De Nayer, a former AA fuel efficiency expert, involved fitting cars with a fuel flow meter and testing them at Millbrook proving ground in Bedfordshire. He found that a Citroën C4 1.6 diesel achieved 99.6mpg at 20mph but only 29.3mpg at 90mph.

The average car consumes 38 per cent more fuel at 70mph than it does over the same distance at 50mph. At 60mph it uses 34 per cent more than at 40mph.

The average driver travelling at 90mph on a motorway will spend £1.20 more on fuel every eight minutes than a driver travelling at 70mph. The 90mph driver will have travelled farther in that time but will still be spending 40 per cent more per mile than the 70mph driver.

The study also found that a driver of the average car travelling 10,000 motorway miles in a year at 80mph would spend £518 more on fuel than if he had driven the same distance at 60mph.

More than half of drivers on motorways break the 70mph limit and a fifth drive at more than 80mph, according to figures from the Department for Transport.

Mr De Nayer said: “There is a huge misconception that the most fuel-efficient speed is around 55mph. The study shows that the slower you go with the vehicle running smoothly, the less fuel you will use.

“By allowing a bit more time for their journeys, drivers will not only protect their licences from speeding points but save themselves a significant amount of money.”

Motorists are also wasting money by using the wrong gear. A car cruising at 40mph on rural roads uses 20 per cent more fuel in fourth gear compared with sixth, the study found. In town, motorists can cut fuel bills by pulling away slowly and smoothly, changing up early and anticipating road conditions to maintain a steady speed.

Mr De Nayer said that improvements by manufacturers to make cars quieter had fooled many drivers into thinking that their engines were running smoothly in lower gears. “They can no longer hear that they should change gear.”

Some manufacturers, including BMW and Audi, have begun installing gearshift indicators to help drivers to improve fuel efficiency.

Greater compliance with the speed limit would spare the environment as well as drivers’ wallets. The UK Energy Research Centre, a govern-ment-funded body, found that if all drivers observed the 70mph limit their vehicles would emit 3.7 million fewer tonnes of carbon dioxide a year. This would be the equivalent of taking three million Ford Focuses off the road.

The Society of Motor Manufacturers and Traders said: “It is unrealistic to expect people to drive much lower than the limit. You need to keep up with traffic and to maintain a safe and reasonable speed.”

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