How Much Extra Fuel Does a Roof Box Use?

A roof box can provide you with an array of benefits, but there's one flaw to installing a cargo carrier to your vehicle. Yep, you guessed it, it's the additional fuel consumption that your vehicle will experience. But how much extra fuel does a roof box use? And are they even worth it?

Roof boxes are notoriously known to be the worst culprit for additional fuel consumption. According to recent reports, drivers spend around 10 to 25 percent more on fuel with a roof box installed due to the extra weight and drag.  

The extra amount of fuel you'll use is exceptionally striking, and even a bare roof rack (without a box or another cargo carrier) can add upwards of 15% to your fuel bill. Therefore, it does add a lot of extra fuel consumption to your travels. But how and why does it affect it so much? We find out below:

How do roof boxes affect fuel consumption?

Hearing the above figures will be shocking for most. Because of this, we thought it'd be beneficial to detail why roof boxes' affect fuel consumption. Once you know why, it undoubtedly makes it a lot easier to figure out innovative ways to reduce this disadvantage of roof boxes.

Airflow

The main reason your fuel consumption will increase when using a roof box is because of the decrease in aerodynamics your vehicle has. Cars, trucks, vans, and other motorized vehicles are designed towards good aerodynamics for a much less restricted and smoother drive.

Without a roof box, as cars travel forward, air must effortlessly travel from the front and over the roof. That's why car bonnets are slightly raised in an upwards shape. It's to allow the air to be pushed up onto the windshield and away from the car.

But what occurs when you add a roof box into the mix? Your car doesn't have such a seamless airflow. Now the airflow travels up onto the bonnet, onto the windshield, and then the roof box. However, when air reaches the roof box, it's restricted.

Because of the restriction felt by the air, your car now must work harder than ever before. This increases the number of RPMs needed to travel, leading to an increase in fuel consumption.

Drag 

Whether you install a rooftop bag, box, basket, or even a trailer, your vehicle will experience drag. Anything that adds weight to your car adds drag. As a result of this, drag is directly linked to an increase in fuel consumption. Practically anything that you install inside or outside your vehicle that is heavy increases fuel consumption. The vehicle has more weight, so it has to work harder to accelerate and maintain speed.

Style

Something else that can affect the increased amount of fuel consumption is the actual style of the roof box. Within the roof box market, there are boxes specially designed for good aerodynamics. However, some designs don't even account for this. In contrast, some boxes are more streamlined. Of course, the streamlined boxes are much better at decreasing the fuel consumption issues you encounter when installing a roof box.  

Positioning

Lastly, the actual positioning of your rooftop box will play a role in the overall fuel consumption. Pretty much anything that alters the aerodynamics of a vehicle will lower its miles per gallon (MPG). When positioning a roof box on your vehicle, you'll want to align the narrowest part with the windscreen if possible. This is because roof boxes that hang over the windscreen reduce airflow even further.

As you can see, there are various reasons why a roof box will affect a vehicle's fuel consumption. However, there are certain things you can implement to reduce the amount this affects your vehicle.

What can you do to reduce fuel consumption with a roof box?

How Much Extra Fuel Does a Roof Box Use

After reading about the increased amounts of fuel consumption you could encounter from installing a roof box onto your vehicle, I'm sure you're eager to find a solidified solution to reduce this from being such a severe problem. Here are three ways to reduce fuel consumption caused by a roof box.

Positioning of the roof box

The first way to reduce fuel consumption with a roof box is its actual positioning on the vehicle. When positioning your roof box, you'll want to keep aerodynamics in mind. The best way to achieve this successfully is to picture rolling a ball along the bonnet, onto the windscreen, and over the roof box. If you're able to accomplish this in one roll, without it catching on anything, you have good aerodynamics.

You want to avoid having the roof box too far back or too forward on the vehicle at all costs. This doesn't allow for seamless airflow and could increase fuel consumption even more.

The choice of crossbars

With all roof boxes, you're required to install crossbars. This secures the cargo box on top of the vehicle. However, sometimes they're also accountable for the increased amount of fuel consumption. These crossbars from Yakima are a great example. They're lightweight, slim, and are specially designed with aerodynamics in mind.

Roof box shape

Like the crossbars, you can purchase roof boxes that follow a similar concept. Instead of just buying any old roof box, you're able to invest your money correctly into a box that's been built to increase the aerodynamics of a vehicle.

In this post, we spoke about a remarkable piece of kit from Yakima, the Rocket Box Pro. This roof box is well-known within the industry for being one of the most aerodynamic cargo carriers currently available on the market. Therefore, it's more than worth considering if it can reduce the amount of fuel you consume.

As you can see, rooftop boxes can increase fuel consumption tremendously. But, there are a small number of solutions that'll reduce this from being so problematic. However, this doesn't entirely cancel out the issues, and it's something everyone has to deal with if they require this additional storage device on their vehicle.

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