By Lori Straus

Have you ever been stuck trying to walk in a rainstorm while holding your umbrella in front in a vain attempt to stay dry? It’s much, much harder to walk, isn’t it? That difficulty in moving forward is reflected in a number called the drag coefficient. And just like your umbrella, your car has a drag coefficient, and the lower it is, the more money you’ll save in fuel costs.

What Is the Drag Coefficient?

The drag coefficient is a number—it has no unit—that reflects how easily something moves through a fluid. (Because we’re talking physics and not chemistry here, air is considered a fluid.)

To put this into carspeak, the drag coefficient tells us how easily a moving car can push air out of the way. Just like an open umbrella slows you down because you have to push more air out of the way to walk forward, a poorly designed car will have to work harder to push air out of the way, too.

Factors That Affect Fuel Consumption

You probably could list right now at least three factors that affect fuel consumption in your vehicle. For example, you might know that powered seats don’t use a lot of electricity but because of their weight can increase fuel consumption by 2-3%. Or perhaps you drive efficiently. You might even be a hypermiling aficionado.

The drag coefficient is another factor to consider when you look at fuel consumption.

Drag Coefficient of a Vehicle and a Vehicle’s Shape

You’ve probably noticed over the years that car design has become more streamlined. In other words, most consumer vehicles don’t look very boxy anymore. Some of this is due to aesthetics, but much of it is meant to decrease a vehicle’s drag coefficient. A cube has a high drag coefficient, whereas a teardrop has a low one.

By decreasing the drag coefficient, car makers are helping vehicles “slip” through air more easily. That reduces the amount of fuel needed to move the vehicle, and the difference shows up in your wallet and in the environment.

Adding Accessories to Your Car

Adding accessories to your car, such as a roof rack or a bike rack, will increase the drag coefficient of your vehicle. That doesn’t mean that you should never buy accessories for your car; the added storage and transportation capacity that accessories can give you make it much easier, for example, to go on family vacations. However, once you no longer need the accessory, remove it from your vehicle. This will help you conserve fuel and therefore leave a few extra dollars in your pocket.

Buying a Vehicle with Fuel Consumption in Mind

In the end, you’ll have little control over the drag coefficient of the vehicle you want to purchase. Focus instead on what you need the vehicle for: city driving, overlanding, family transportation, etc. In addition, look up the EnerGuide for the vehicle you’re interested in. It will show you fuel efficiency ratings when the vehicle is new. Buying a hybrid is another way to save on fuel.

Considering these overall factors will help you find the vehicle best suited to your needs and one that helps save money on fuel.