Road tests raise questions over small car fuel economy claims

Since the dawn of the motorcar it has been common knowledge that vehicles with smaller engines and less weight are less thirsty at the petrol pumps and represent a greener alternative for motorists. But recent research by Emissions Analytics has revealed results that some might find surprising.

In testing 500 vehicles (half petrol and half diesel) for three hours on UK roads, it was found that the average car travelled 18% fewer miles per gallon than claimed under manufacturer specifications. More concerning still were the findings that these mpg claims were more inflated in smaller cars than their more powerful counterparts – the average 1.0litre engine car was found to be 36% less efficient than advertised, for example.

In the test, the average fuel consumption of cars with a 1.0–2.0litre engine was a 46.7mpg, as opposed to the average advertised rate of 59.1mpg.

And 2.0–3.0litre vehicles? They reality for these larger vehicles was only 15% below that advertised by the manufacturers.

One theory to explain this is that less powerful engines require more energy when accelerating, draining fuel and impacting on fuel consumption claims. While accelerating and decelerating is a regular occurrence in the real world, it plays only a small part in the official test conditions.

This is food for thought for motorists – both private and commercial – who choose to make purchases based on the economy figures supplied by manufacturers.

One line of thought would be that the test conditions used to gain these figures are a far cry from the reality of driving on British roads. Alternatively, it is possible that the inflated figures for small cars is due to manufacturers’ awareness that consumption is more crucial to a sale in small vehicles and less of a concern for buyers of 4x4s and executive saloons.

While it has been common practice for cost-aware and environmentally conscious drivers to take on smaller cars in the past, these findings suggest that the benefits of smaller engines may not be as significant as once thought.

As a representative for Emissions Analytics stated: ‘For maximum fuel economy you should look for a one to three-litre engine, as these will return around 45-46mpg, and to avoid being too disappointed with the result, pick a two to three-litre vehicle as it will be only 15 per cent worse than you were told you could achieve.’

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