Will A Car Roof Box Affect My MPG? – Improve Your MPG!

car roof box mpg

If you’re fitting a car roof box to your vehicle you might’ve wondered whether it will affect your fuel economy or MPG…

A roof box is typically a hard shell container that secures to your car’s roof and allows you to store extra luggage.

They’re great for travelling, allowing you to carry extra gear and equipment that you might not be able to without one.

The question comes down to whether a roof box will really affect your MPG.

What Is A Car Roof Box?

Have you ever gone on a road trip packing your car’s boot or back seat to the brim full of gear?

Does that sound a lot like you?

You’re not alone, and if you’ve got a smaller car you’ll likely want to try and fit as much as possible into the car…

A car roof box can help you out by allowing you to store all this extra equipment in a separate storage item allowing for additional space in your car.

More breathing space for you and your buddies.

You can purchase a car roof box online for a reasonable price, I suggest checking out my review on the best car roof boxes here.

Will A Car Roof Box Lower My MPG?

The short and sweet answer is yes.

It will lower your fuel economy.


Because you’ve strapped a roof box to your car’s roof that will affect the airflow of your car making your car less fuel efficient by creating more “drag”.

“Drag” is where when the air flows over your car it ends up hitting and/or sticking on something meaning it doesn’t flow smoothly.

Not to mention that they’re pretty bulky items anyway.

I guess the next question really is whether you’ll actually notice the lower mpg?

And that chances are, you won’t.

If you did notice the slightly lower mpg, chances are that it’s not really going to cost you that much extra over a longer commute – if the cost of fuel is important to you.

Ways To Improve Your MPG

There are a few things that you can do to improve your car’s mpg to gain back the lost mpg caused by your car’s roof box.

Keep Your Tyres Inflated

One way that you can make sure that you’re getting the best possible mpg with a car roof box fitted to your car is by making sure your tyres are inflated correctly.

And this doesn’t mean whether they’ve got “enough” air in them.

You want to make sure they’re at the correct pressure or PSI.

If you’re unsure or don’t know what pressure your car tyres need to be at, the best way to check is through your car’s handbook.

It could be around 30 – 35 PSI for 5 door hatchbacks.

Make sure you check whether there’s a different PSI rating for carrying different amounts of weight as your roof box will be adding extra weight to your car.

It could be the same amount of weight as 2 people for comparison.

Also, whilst you’re checking your tyres – are they in excellent condition? If not, you might want to look at getting some new tyres fitted.

Not just any old tyre, you’ll want some good quality ones – I’ve got a blog post that’ll help you with knowing whether cheap tyres or expensive tyres is the route to go!

Reduce Car Roof Box Drag By Keeping Your Windscreen Clean!

Keeping your car’s windscreen clean is so important!

It’s important to your safety and the safety of others on the roads, so keeping your car’s windscreen clean acts as a bonus to reduce the drag caused by your car roof box.

In turn, it should provide you with a better mpg to counter the drag caused by the roof box.


As air flows over your car’s windscreen it’s forced up over your car’s roof.

If the windscreen isn’t kept clean, it will create additional drag.

Fitting A Car Roof Box To Your Car

Did you fit the roof box to your car?

Or is it still waiting to be fitted by either you or someone else?

You should make sure that your roof box is fitted and secured to your car correctly.

A moving roof box whilst you’re driving will not only affect your mpg, but could come lose whilst you’re driving and even potentially fall off!

I’ve got a guide on fitting a car roof box if you’re stuck and need some direction with getting it onto your car.

Will A Car Roof Bag Reduce My MPG?

I’ve given you a lot of pointers so far with improving your mpg whilst you’ve got a car roof box fitted.

However, you might actually have a roof bag instead?

If this is the case then great!

Because chances are, your car’s roof bag will be smaller, and potentially a little less heavy if you’ve not overloaded it.

A car roof bag likely will get you a better mpg when compared to a hard shell roof box thanks to the smaller size.

If you can try, shape the roof bag to your car’s roof design so that airflow can flow more freely over the bag.

If you’re able to do this, expect a slightly better mpg – how much? We’ll cover that next!


If you’ve read any of my points above to improve your car’s mpg when you’ve got a car roof box fitted then hopefully you’ll see an increase in your mpg figure.

Excellent news!

But how much would all of these tips actually equate to in real figures?

How much will you actually be improving your mpg by?

Well, here’s the thing… you’re not going to improve your mpg by a great deal, it’s likely only going to be between 0.2 and 1.0 mpg better, any more and that’s a bonus, but don’t expect too much.

Ultimately, your roof box will reduce your mpg as it’ll create drag, but follow my tips and you should start seeing marginal results soon.