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What is a coefficient of drag, and why should I care?

By Tony O’Kane, 28 May 2019 Car Advice

What is a coefficient of drag, and why should I care?

It’s a number that car manufacturers frequently brag about, but what does it mean and why is it important?

Flip through a car brochure or scroll down a manufacturer’s website, and you may come across a mysterious metric that, judging by the boastful language surrounding it, certainly seems important: the coefficient of drag.

That last word will probably clue you in a little bit, with “drag” obviously signaling that the number has something to do with a car’s aerodynamic performance, but the number itself can often seem mysterious.

Usually if a carmaker is bragging about it, it’s in the low region, somewhere between 0.26 and 0.22 – and the lower the number, the better. For reference, an aerodynamically-ideal teardrop shape has a coefficient of drag of around 0.04.

But what do those digits actually mean?

toyota prius wind tunnel

The coefficient of drag, usually abbreviated to Cd or called the drag coefficient, is not a measurement of any one thing, but rather an expression of a car’s total drag divided by its frontal area.

The former means the total amount of wind resistance generated by the car’s shape and surface when moving at a certain speed, while the latter is the total area of air displaced by the car as it moves forward – view it from the front, and you’re essentially looking at that area.

It’s a relative measurement, which means two vehicles with the exact same Cd don’t necessarily create the same amount of drag. It’s entirely conceivable to have a bus with the same Cd as a passenger car, but the bus - thanks to its greater frontal area – will generate significantly more drag as it moves through the air.

A low Cd doesn’t necessarily mean low drag. Size, as always, matters.

Here’s an example: the Tesla Model X and first-generation Honda Insight are two very slippery cars with each boasting a Cd of 0.25, but the significantly larger Tesla creates more drag overall. Bear that in mind when you see manufacturers bragging about Cd figures for large SUVs.

So why should you even care about that number? Here’s the thing: aerodynamic drag has a massive effect on a car’s fuel efficiency at highway speed, and that effect increases exponentially the faster you go. A car with a low Cd has the ability to slip through the air more easily, burning less fuel as a result and saving you more money in the long run.

That said, always consider it in the context of the car’s size. If you’re thinking that a large SUV with a low-ish Cd value is going to save you more fuel compared to a smaller wagon with a slightly worse Cd, think again: the opposite is more likely to be true.

One might also think being able to slice through the atmosphere with ease at high speeds would mean a low Cd figure is also important for performance cars, but that’s not correct.

Quite the opposite, in fact – sports cars typically have quite poor drag coefficients, thanks to their use of downforce-generating, lift-reducing or airflow-disrupting features like diffusers, wings, spoilers, vortex generators and cooling vents. They all get in the way of clean airflow, but fuel economy is rarely a priority for a performance car so it doesn’t really matter anyway.

On the other hand the bodywork of efficient cars, like the aforementioned Tesla Model X, the Toyota Prius, Hyundai Ioniq and more, have been designed with the sole aim of reducing drag as much as possible. So, if you’ve ever wondered why a Prius has such a weird hunchbacked silhouette, now you know – it’s all in the aid of producing the lowest coefficient of drag  possible.