We often highlight the benefits of clearing out your car’s boot and cabin of unnecessary weighty items, and optimising your car’s aerodynamics by taking off roof bars and roof boxes and cycle carriers when they are not being used. Both undoubtedly will save you some fuel, but the actual gains are individual to any given car, its engine, its shape, its weight, and how it’s driven. So what sort of figures are possible, and which areas might offer the biggest potential gains?

Much research has been done, particularly by manufacturers, in seeking to reduce the fuel consumption of their cars ñ not just as a selling point, but because they have had to meet CO₂  targets for their model ranges. When new cars are launched, often with figures quoted for the weight reductions derived from use of lighter suspension parts (typically aluminium in place of steel) and lighter body panels (often thinner, high strength steel, replacing heavier conventional steels), these are often accompanied by engine changes and other efficiency gains. Investigate things closely, and it’s evident that any claimed fuel economy gains are coming more from engine and driveline changes, or aerodynamic improvements, than from any simple weight reductions. Whilst weight reduction is rightly targeted by manufacturers, the potential gains from throwing unnecessary junk out of the boot in our daily motoring are, somewhat disappointingly, rather minimal.

There are many rules of thumb for the real life fuel economy gains achieved by weight reduction, and they should really differ for the type of car involved; but they all come down to the order of half of the percentage weight reduction being returned in percentage gains in fuel economy: so a weight saving of 50 to 60kg, or four to five per cent of a small hatchback’s weight, might offer a two per cent plus gain in fuel economy, dependent on the type of motoring in which you indulge. Putting it in perspective, but in reverse, adding the weight of three heavy adult males in the back seat of your car, thereby increasing its weight by twenty per cent, might at worst drop its fuel economy from 45mpg down to around 40mpg in mixed motoring, and probably only down to 43 to 44mpg on the motorway. In fact there’s something pulling in the opposite direction, because the heavily laden car sits lower to the ground, and is therefore more aerodynamic than with only its driver aboard. All other things being equal though, the weight factor is reflected primarily in the energy used to accelerate your car, and to raise its altitude when climbing any hill, or mountain. Little of that energy is recovered on slowing down, and little of the energy used to climb a hill is regained when going back down the same hill. Some of that lost energy can be recovered with hybrid and electric cars, using energy recovery systems, but in conventional vehicles, motoring in hilly country and driving enthusiastically inevitably uses more fuel, as weight is then a more significant factor.

So hard-driving owners living in hilly country should see the biggest gains from taking all that clutter out of the boot. But the typical family car has evolved significantly over the last decade or more and, looking back, ranges like the Peugeot 207 and the Mk 5 Golf looked distinctly overweight, and their successors were deservedly much lighter. But many families now enjoy the flexibility of physically larger MPVs and SUVs when, in past times, they would have owned smaller hatchbacks. In reducing vehicle weight, the manufacturers have enabled the use of smaller, more efficient power units in bigger vehicles. Without the increasing use of weight-reduced body and engine parts, and more efficient engines, we would never have seen 1.6-litre diesel engines offering more than satisfactory performance in seven-seater cars like the Volkswagen Touran and Vauxhall Zafira Tourer, or the CitroÎn Grand C4 Picasso which, at under 1,350kg, weighs less than many hatchbacks from five years ago.

So, whilst we are undoubtedly gaining from having lighter cars, the potential for owners to save fuel by exploiting weight reduction is distinctly limited ñ not that we should ever scorn that possibility and the benefits of keeping a tidy cabin and boot. Next month we’ll look more deeply at the fuel economy gains from good aerodynamics, and the part played by tyre rolling resistance.

Victor Harman