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Does driving more slowly really make a difference to fuel consumption?

23 Jan 2013 at 10:38

fuel-consumption fuel-economy

There’s great debate as to whether driving slower makes a difference – if cutting your speed helps improve economy at the expense of journey times, or if it’s all just an urban myth.

One thing is for certain, driving slightly slower will give you more time to react to unfolding events, as well as an increased distance in which to brake to a stop. So it has to be good for safety, at least.

However, there is evidence to suggest that reducing your speed can help reduce your fuel consumption, too.

According to fuel-economy.co.uk travelling at 80mph on the motorway – other than breaking the law – means increased fuel burn, with your engine slurping between 10 and 20% more fuel than if travelling at the legal limit of 70mph.

Yet doing this increased speed over a 20-mile motorway commute for example, will only shave two minutes off your journey time.

Extrapolate that over an average 12,000 miles over 12 month’s worth of motoring and it could cost you an extra £1,365 in fuel – especially if you do plenty of motorway miles.

And that’s just based on a normal car, such as a 1.6-litre turbodiesel Ford Focus that returns 67.3mpg combined, and today’s average diesel price of 140.4p per litre.

Of course, it stands to reason that in top gear in any car, 80mph will mean your engine is turning faster – and therefore burning more fuel – than at 70mph, so it’s perhaps easy to see why fuel is saved.

The big surprise for many is actually the difference in journey times between driving at 80mph and 70mph. As in, the difference is nowhere near as great as they expect in the real world.

We’ve noted the claim that under ideal traffic conditions your journey time will only increase marginally – but out on the open road, even this disparity in journey time vs speed might not be as great.

Managed motorways – featuring overhead gantries that display temporary speed limits and house speed cameras to snap those who contravene the short-term rule changes – temporarily reduce traffic speeds in a bid to ease congestion. In general it works, too.

At busy times, employing this logic can help you save fuel while not actually increasing your travel time.

Instead of racing quickly up to a queue, having to brake hard then accelerate again, keep a constant, lower speed, predict traffic conditions and you could find your fuel tank fuller than you thought – without arriving home any later.