A roofbox does change the airflow over a car and as such, it will have a very small impact on your fuel economy. As the vehicle will essentially become larger, there is more wind resistance and due to this the car will have a slightly reduced amount of miles per gallon – this is known as “drag”.

As well as the additional wind resistance, the roofbox will allow you to carry more weight due to the belongings inside it.

Both of these combined will cause your engine to work a little bit harder to move the car as normal.

Does that mean your fuel consumption will go up massively? Well according to most manufacturers, fuel consumption wont dramatically increase. Yes it will go up slightly due to the additional drag and weight but not significantly enough for you to notice much difference.

When speaking to Thule who are one of the biggest and most advanced on the market, they estimate that you should see no more than 5% increase in your fuel consumption with a roofbox attached.

You will notice nearly all roofboxes are a strange shape, this is by design to help the roofbox cut through the air resistance and reduce drag – you will notice the front section is usually flatter – like a wedge shape – and this means a smoother air flow over the car.