Air, your public enemy
Of course I am not taking you to academic depths here; the movement of air is rather chaotic and hard to describe. However, there is one function describing the relationship of air resistance and the energy it absorbs that is worth learning about. It is namely this force your engine uses most of its output to fight, ie. to counteract the forces acting in opposition to the intended direction travel. There are many such forces, from mechanical friction within the drivetrain to the rolling resistance of tyres, but none are as powerful as drag. And because it is single-handedly responsible for the better part of the vehicle's energy consumption, hence fuel consumption, it is also here you can save the most.
So what can we do about it, you may ask. The car's design is a set matter, you can make your choice when opting to purchase it or not, but you cannot influence its drag in any ways during use – or can you? Air resistance depends on four factors, only two of which are constant, the other two being dependent on conditions of operation. These four factors are the density of the medium (air), the frontal area, the speed of air flow, and the coefficient of drag (customarily denoted Cw, Cd, Cx or sometimes Cz) relating to the object - in this case, the car.
Here is the formula
F = v2x cWx A x 1/2 ρ,
F is drag
v is speed
cW is coefficient of drag
A frontal area
You cannot influence the density of the medium; for all practical purposes it is constant at ground level. Of course air gets thinner as your elevation increases but you really cannot capitalise on the advantages of this unless you are flying an airplane – by climbing to over 33 thousand feet airliners burn a lot less fuel. But if you drive your car to higher altitudes you will lose whatever you might gain on drag due to power losses caused by the thin air and the need to drive uphill.
Frontal area is also a given value but it's something you should be aware of. A typical sports utility vehicle (1.9 metres wide by 1.8 metres tall) has a 50% larger frontal area than a typical small car (1,7 by 1.4 metres), although they may have identically efficient powertrains. This difference boils down to real life savings. Between vehicles with similar power to weight ratios and identical types of powertrain, a passenger car will have a mileage that is 30-40% better than that of an SUV. Of course there are other components to this but higher drag does play an important role here. Strictly financially speaking, then, it does not pay to operate a car larger than what you absolutely need because the larger it is, the more costly it will be to maintain.
Coefficient of drag – abbreviated to Cd in English terminology and Cw in German context – is slightly less straightforward. It refers to how an object interacts with the air flowing around it. The easier it is for air molecules to move around an object, the lower the coefficient of drag is. However, because this also depends on the physical ratios as well as the surface quality of the object, Cd cannot be calculated but has to be measured every time. A plane sheet set up perpendicular to the airflow will have a Cd of around 1; an elongated cuboid around 0.8; a sphere around 0.3-0.5; while an aerodynamically perfect raindrop shape will be around 0.2-0.05.
However, cars are not homogeneous bodies, and minor details are just as important for shaping drag as the overall shape. You can ruin the drag of a raindrop-shaped car by including too numerous gaps or protruding components disrupting the smooth flow of air and creating pockets of turbulence. That is why you can have similar coefficients of drag for a boxy and a more rounded vehicle – in such cases, designers of the rectangular car paid closer attention to details. It is also due to the many turbulence-inducing parts that there has been very little progress in this field in the last twenty years or so. Cd values of 0.29-0.30 became the order of the day by the 90's, and the 1989 Opel Calibra is still among the very best car bodies for drag (Cd 0.26). Designers are now down to minute details like the uniformity of the base plate, panel joints at doors, air twirl induced around the wheels and so forth. This means, however that in order to further improve Cd values you would need more efficient cladding, and higher precision manufacturing, both of which would make mass manufacturing of cars excessively expensive.
So it takes a lot of effort and resource to improve coefficients of drag – the Mercedes-Benz CLA is now leading the mass market pack with a Cd value of 0.22 –, but of course you can always take a good value and wreck it. Remove your bumper, break your wing mirror cover or leave the roof rack up and you can add costly tenths to the Cd value of your car, something that will also influence your actual fuel consumption. For full detrimental effect you can always add popular accessories like spoilers or wings. These of course serve the useful purpose of creating downforce on racing vehicles, but they also botch up the aerodynamics of the given vehicle epically. Formula race cars have a Cd value of 0.7 to 1.5, depending on the geometry and angle of said wings – worse than your typical tractor trailer. These aerofoils produce tangible results on race cars that are run at the highest possible speeds but on the street they hardly serve any purpose other than aesthetical: you would likely not feel any added stability driving slower than 120-150 kph, while fuel consumption will likely suffer, losing tenths of litres over 100 km. In short, aftermarket spoilers are generally speaking a waste of energy and resources on street cars.
However, the effects of these three components are insignificant compared to the ultimate factor of influence – speed. The reason is that it is represented twice (counting drag force) or even three times (considering power) in the formula. More precisely, drag force is proportional to the square of speed, while the power needed to overcome drag is proportional to the cube of speed. What this means, among other things, is that you need a disproportionate amount of power to achieve high speeds. Consider an average car with a 120 PS engine and a top speed of 200 kph. In order to reach 300 kph this car would need about 420 PS, and to go up to 400 kph, as much as 1000 PS. Now, can you guess how much power it takes to maintain a speed of 100 kph? 16-18 PS, approximately! Here's a lifelike example: you try to accelerate from a steady 50 to 63 kph. Doing so you double the power needed to counteract air resistance, and along with that, you increase fuel consumption. You can safely say that increasing your speed by 25% doubles the amount of power consumed by drag.
I am not saying you should be driving like a sloth. But do remember that power demand and fuel consumption increases at a much higher rate than speed. Driving at 130 kph your engine will be running at 65% higher load than at 110 kph, all other factors being identical. It means that a dash of sensible thinking can save you significant amounts of fuel without compromising your travel time too much. You can also improve your mileage by removing all protruding components that are not being used. It may take as much as fifteen minutes to remove a roof rack but it can save you a few Euros on your next fill-up.