Do Cargo Boxes Affect MPG?

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A roof box can provide you with various benefits, but there's one flaw to installing a cargo carrier to your vehicle. You guessed it; it's the additional fuel consumption your vehicle will experience.

Generally speaking, Roof Cargo Boxes have a minimal effect on the fuel efficiency of a car due to their aerodynamic design.

A study found that a sedan with an empty roof rack would get impacted by 11 %, and rack and roof cargo boxes would impact by 19 %. However, small SUVs with racks only will impact by 2%, and roof cargo boxes with roof racks will impact by 13%.

According to the research, The empty roof rack reduced fuel efficiency by nearly 2 percent, equating to approximately three-quarters of a gallon of gasoline. Mileage fell from 41 mpg at 65 mph to 40 mpg (13%). With the cargo box installed, fuel economy decreased by 13 percent to 35 mpg.

How to make roof racks more aerodynamic

There are several ways to make a roof rack more aerodynamic. The most common is to use a wind fairing attachment on Amazon or aerodynamic-specific crossbars on Amazon. These can be reduced through investing or making DIY equivalents of these items. I wrote a whole article on making roof racks more aerodynamic check it out here.

The extra fuel you'll use is exceptionally striking, and even a bare roof rack (without a box or another cargo carrier) can add up to 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 easier to figure out innovative ways to reduce this disadvantage of roof boxes.


Your fuel consumption will increase when using a roof box 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.

Air must effortlessly travel from the front and over the roof without a roof as cars travel forward. That's why car bonnets are slightly raised in an upwards shape. It allows 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, the windshield, and 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.


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, drag is directly linked to increased fuel consumption. Practically anything that you install inside or outside your vehicle that is heavy increases fuel consumption. The car has more weight, so it must work harder to accelerate and maintain speed.


Something else that can affect the increased amount of fuel consumption is the actual style of the roof box. There are boxes specially designed for good aerodynamics within the roof box market. 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. 


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). For example, when positioning a roof box on your car, you'll want to align the narrowest part with the windscreen. This is because roof boxes that hang over the windscreen reduce airflow 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 car.

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

After reading about the increased 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. So, 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. Therefore, you'll want to keep aerodynamics in mind when positioning your roof box.

The easiest method is to imagine rolling a ball down the bonnet, onto the windscreen, and across 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 forward on the vehicle. Unfortunately, this doesn't allow for seamless airflow and could increase fuel consumption.

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 fuel consumption. These crossbars from Yakima are a great example. They're lightweight, slim, and specially designed with aerodynamics in mind.

Roof box shape

Like the crossbars, you can purchase roof boxes that follow a similar concept. However, instead of just buying any old roof box, you're able to invest your money correctly into a box 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 for being one of the most aerodynamic cargo carriers currently available. Therefore, it's more than worth considering if it can reduce the amount of fuel you consume.

Cargo box capacity

Unfortunately, the final tip on reducing fuel consumption with a roof box is the most difficult to implement. Therefore, this tip is only relevant if you don't need to use a roof box frequently.

If you only require a roof box for specific journeys or trips, it might be beneficial to purchase a smaller model. The increased drag from a larger roof box will have a more significant impact on fuel consumption.

For example, the Yakima SkyBox 16s is an excellent option if you're only looking for a roof box to use occasionally. It has a 16 cubic feet capacity, which should be more than enough for those shorter journeys.

On the other hand, the Thule Motion XT XXL has a massive 22 cubic feet capacity. This is one of the largest roof boxes available and, as a result, will have a greater impact on fuel consumption.

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.

About the author 

Vieira Babeker

Hi! I'm Vieira. I've always been interested in different Auto Accessories

My passion here at is to provide helpful straight forward clear advice and tips for Automotive Accessories.

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