Not so fast: why going slower can save you a packet on petrol

theguardian.com:

When Turlough Downes left the roof box on his car after a holiday, he knew it would push up the cost of fuel. However, after he did the maths, he was stunned to find out by just how much … a lengthy journey across Ireland had cost 50% more in fuel.

“It really caught me off guard. It wasn’t what I expected. I expected it to make a small difference, but I was really shocked,” he says.

Downes, professor of mathematics and astrophysics at Dublin City University, has applied his knowledge of aerodynamics to fuel efficiency, to understand how drivers can save money.

There are many reasons for cars to use more fuel and drive up the cost of motoring – speed, weight, how the gears are used among them – but many drivers will be surprised just how much money can be saved from slowing down and making small changes when behind the wheel.

The reason for the jump in cost when you use a roof box is that the added weight and air resistance means more fuel is used to move. Curious to see how much more drivers who sped past him were spending on fuel, Downes has also examined the effects of slowing down.

Motoring groups such as the RAC have told drivers they can save money if they drive at lower levels than the speed limit. This typically applies when travelling a considerable distance where they can reach high speeds, such as on motorways.

The differences are significant, according to Downes.

A driver on a 45-mile journey from Manchester to Leeds, using an average car, can save around £4.30 if they go at 50mph, rather than the national speed limit on a motorway of 70mph, using the current average price of a litre of petrol at £1.62.

The savings are similar whether using petrol or diesel.

On long journeys, the most important issue for drivers is air