The Extra Mile

The Extra Mile

Let us return again to aerodynamics, and how they can significantly affect your fuel economy. The inescapable laws of physics determine the amount of energy required to push a car through the air, which is dependent on the shape and size of the car (so-called ìdragî or air resistance), the density of the air, and the square of the speed of the car. The significant effect of speed is illustrated by comparing drag figures for 50mph and at 70mph, when it is almost doubled. Aerodynamic drag therefore becomes the major energy consumer of engine power (up to which the rolling resistance of the tyres is greater) somewhere in excess of around 30 to 40mph, and it uses more than 80 per cent of the engine power required when speeds reach around 60 to 70mph, depending on the body shape and size. On paper, slowing down from 75mph to 65mph reduces the power needs by around 30 per cent and, whilst the effects vary with the car’s gearing and engine power delivery characteristics, it therefore uses approximately the same amount less fuel.

What we can observe about motoring today is that the continuing market popularity of SUVs and crossovers means that the frontal area of cars is inevitably increasing, as well as vehicle weight. Marketing blurb claiming good aerodynamics by quoting any such car’s impressive Cd, or drag coefficient, evades the fact that the overall aerodynamic drag of SUVs, crossovers and full-blooded off-road 4x4s is higher than that of hatchbacks or saloons, simply by virtue of this increased frontal area. And such cars are inevitably heavier than comparable hatchbacks and estates, use more fuel when accelerating, and waste more energy in braking. So, you cannot expect to change from something like a Golf or Passat Estate to a Touran or Tiguan, or from a Focus Estate to a C-MAX or Kuga, without paying some penalty in higher fuel costs. By their nature, such increasingly popular cars with off-road pretensions, as well as being taller, also inevitably have higher ground clearances and complex suspension systems, which leads to further increased aerodynamic drag generated in the area between the underfloor of the car and the road surface. Compare a conventional Volvo V90 D5 AWD and a XC90 SUV with the 232bhp D5 engine, and you’ll see that the V90 is over 200kg lighter and 8mpg less thirsty!

Whilst you therefore will inevitably pay at the pumps for the undeniable practicality and space of SUVs, the higher seating position and easy access and egress, there’s plenty of scope for minimising the downsides, and here are some thoughts that you can apply. Firstly, if you’re planning to cover significant distances with bicycles on day trips or short breaks, see if you can fit them into your cargo area (quick release wheels will help), rather than using a rooftop cycle carrier, when the increase in fuel consumption can be significant ñ and the cycles will also stay dry and clean. The same goes for taking off roof boxes when you are not using them. Rear-mounted cycle carriers are less drag-inducing than rooftop carriers, although the turbulent air around a tailgate will often coat your cycles with a thick layer of filth on dirty roads!

As for driving techniques, we’ve probably said enough about the speed factor, but, if you’re mixing it with big HGVs on motorways and dual-carriageways, don’t follow them too closely, as attempts at slipstreaming them are always a dangerous pursuit. Thatís because the area following such large vehicles is quite turbulent for as much as 50 yards behind, and you’ll use more fuel cruising there than when you’re further back in ìcleanî air. You’ll also see more of the road ahead and avoid unnecessary braking, even if other ignorant drivers keep sneaking into the space that you are trying to leave clear.

Victor Harman 

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