According to research commissioned by What Car Magazine, modern cars are most fuel efficient at between 20mph and 40mph at a steady speed, not the traditional 56mph which is widely believed. Tests on five different vehicles were carried out ranging from a 1 litre Toyota Aygo to a 2.2 litre Land Rover Freelander. The most efficient speed was below 40 mph for all five and as low as 20 mph for two of these.
The Citroen C4 1.6 diesel achieved 99.6 mpg at 20 mph.
Reference: Slower diving can save £500 a year
Our own observations from on-board computer readings is that a BMW 320d will return in excess of 50mpg at both a steady speed of 20mph and a steady speed of 30mph. However the fuel efficiency can drop dramatically and to as little as 10mpg during acceleration between 20mph and 30mph. It is accelerating that is the most demanding on fuel consumption, whereas maintaining a steady speed uses comparatively very little fuel.
Urban driving is rarely about steady speeds, instead it is a complexity of speeding up and slowing down, and waiting at junctions or at traffic lights. When driving in urban areas, every time 30mph is attained, very soon it will be necessary to slow down again and so the fuel that was used to achieve 30mph will have been wasted.
Driving at a slower, but more steady speed and avoiding accelerating where possible will increase fuel efficiency, leading to lower costs, lower emissions, cleaner air and less of an environmental impact.