Aerodynamics: It’s a drag – Inside the confusing science of highway fuel efficiency

Carmakers compete on thousands of levels, with fuel efficiency increasingly towards the top of the competitive tree. Any self-respecting automotive spec sheet will claim a value called C_D, which is laboratory shorthand for ‘coefficient of drag’.

Most of us know it’s related in some way to aerodynamic drag (or slipperiness in air) and therefore related to fuel efficiency – but most of us are a little hazy on the specifics.

Here’s how it works.

Air is pretty thick stuff. If you’ve ever ridden a motorcycle and stuck your head up, clear of the cowl, at 60mph, you quickly figure that out pretty quickly. Because we breathe it every day, however, it’s easy to forget it’s even there. More on the properties of air

There are two ways to reduce how hard it is to push a car through the air: you can just make it smaller, or you can improve the shape. Half the frontal area means half the drag (or twice the slipperiness; same thing). Unfortunately, it’s impractical and uncomfortable to fit in a half-sized car, which is why carmakers spend so much time optimizing shapes inside wind tunnels. (The other reason is to banish noise at highway speeds, but that’s another story.)

At the end of the day, the new car is produced with the final shape being a product of: a) the dictates of fashion and manufacturing limitations, b) the various regulatory compliances required (pedestrian protection, light placement, etc.) and c) aerodynamic slipperiness. A number appears in the spec sheet, in the C_D column.

C_D is really a comparison. A car with a really bad C_D of, say, 0.5 is half as hard to push through the air as a rectangular plate with the same surface area. A car with an outstanding C_D of 0.25 requires only a quarter of the effort required to push a rectangular plate with the same area. More on determining the drag coefficient

Carmakers use C_D as a basis for competition: ‘ours is slipperier than theirs’. However, C_D only tells you half the story. Actual frontal area is the other half. Big cars are still harder to push down the highway.

When you’re powering down the Interstate at 60 or 75mph, aerodynamic drag is about half (or more) of the total resistance that the engine must overcome. (Other resistances include: rolling resistance of the tyres, driveline friction, internal friction inside the engine, compression of the next fuel/air charges, and driving engine ancillaries like power assisted steering, air conditioning, and the car’s electrical system.)

The amount of drag depends on four things: the density of the air (how thick it is), the car’s speed (squared), the C_D and the frontal area. Bottom line, you can’t change the first one – the air is how it is, wherever you are. You can change your speed, and this will have a profound effect on drag. The last two variables, however, are the ones intrinsic to the car itself. Unfortunately, carmakers don’t tell you the frontal area.

If you’re wondering how much power it takes to push your car through the air at any speed, you need only to add basic high school physics. Drag force x velocity (speed). More on actually calculating the drag numbers

The easiest way to calculate it is to calculate the drag and then simply multiply by V again and you’ll get the power. In the Imperial system you’ll get horsepower; in the metric system you’ll get watts. More on figuring out the power

There’s a few interesting observations inside these numerical relationships.

Doubling your speed quadruples the load on the car, because drag is proportional to speed squared. Twice the speed; four times the drag. If you compare a car at 35mph with one at 70mph, it’s four times as hard to push it through the air at 70mph.

Then there’s the matter of engine power required to overcome the air resistance. Power is proportional to the cube of speed, so if it takes 20hp to push the car through the air at 50mph, it will take eight times that – 160hp – to overcome drag at 100mph.

What this means is that travelling speed has a huge bearing on highway fuel consumption. It always will have, and clever engineering cannot subvert the relationship – unless they can figure out how to thin the air around the car…

The other thing to bear in mind is that C_D isn’t the whole story when it comes to how slippery your car is. Some future Toyota Prius with a hypothetical C_D of 0.3 will always be easier to push down the Interstate than a future Dodge Ram with a hypothetical C_D of 0.3 – simply because the Dodge has a much greater frontal area. It’s only ever OK to compare C_D values of cars that are roughly the same size, otherwise the comparison is meaningless.