Driving With a Roof Box: Speed Limits, Tyre Pressure and Fuel Use Explained

A roof box can be a lifesaver when you’re packing for a family holiday. Instead of playing a stressful game of luggage Tetris in the boot, you have extra space for suitcases, camping equipment and all those “just in case” items that are always included. 

However, adding a roof box changes more than your car’s storage capacity. It increases the vehicle’s height and weight, affects its aerodynamics and can alter how it feels on the road. 

Before you set off, here’s what you should know about the roof box speed limit, tyre pressures, fuel consumption and safe loading. 

 

Is there a roof box speed limit in the UK? 

There isn’t a specific legal speed limit for driving with a roof box in the UK. You must follow the speed limit displayed for the road and your vehicle, which means the usual maximum of 70 mph on motorways for most cars. 

However, that doesn’t automatically mean 70 mph will always be the safest speed. 

Your roof box or roof bar manufacturer may recommend a lower maximum speed, so check the instructions before travelling.  

You should also reduce your speed when driving in: 

• Strong or sudden crosswinds  

• Heavy rain or poor visibility  

• Busy traffic  

• Areas with sharp bends  

• Unfamiliar road conditions  

A roof box gives the wind a larger surface to push against. You may notice this most when passing large vehicles, travelling across exposed bridges or emerging from sheltered sections of road. 

If the car feels less stable than usual, ease off the accelerator and leave more space between you and the vehicle ahead. 

 

Does a roof box change how your car handles? 

Even a properly fitted roof box raises your car’s centre of gravity. This can make the vehicle feel slightly different when cornering, braking or changing lanes. 

The difference may be subtle, but it’s still sensible to drive more smoothly than you usually would. Avoid sudden steering movements, accelerate gradually and allow extra time to slow down. 

Remember that your car is also taller. Make a note of its total height before travelling, particularly if your journey involves: 

• Multi-storey car parks  

• Height-restricted entrances  

• Ferries or Eurotunnel terminals  

• Hotel car parks  

• Low branches or barriers  

It’s surprisingly easy to forget the roof box is there after a few miles. A note on the dashboard can provide a useful reminder before you enter somewhere with restricted clearance. 

 

What should your tyre pressure be with a roof box? 

When using a roof box, inflate your tyres to the manufacturer’s recommended “laden weight” or “full load” setting. You can learn more about checking your tyre pressure in Merityre’s helpful guide. 

Check the pressures before setting off, while the tyres are cold. Checking them immediately after a long drive can produce an inaccurate reading because the air inside the tyres expands as it warms up. 

It’s also worth checking them again before your return journey, especially if the car has been parked for several days. 

 

Why tyre pressure matters on a heavily loaded car 

Incorrect tyre pressure can affect your car’s handling, braking and fuel efficiency. An additional load can place more strain on tyres that are already underinflated, while overinflated tyres can offer less grip and can wear unevenly. 

Before a long journey, check all four tyres for: 

• The correct pressure  

• Cuts, cracks, lumps or bulges  

• Uneven wear  

• Objects embedded in the tread  

• Sufficient tread depth  

If you’re unsure what to look for, Merityre explains how to inspect the condition of your tyres and check your tyre tread depth. 

Don’t rely solely on the tyre pressure monitoring system. It can warn you about a significant change in pressure, but it doesn’t replace regular manual checks. 

 

Does a roof box use more fuel? 

Yes, driving with a roof box usually uses more fuel. 

The box increases the car’s aerodynamic drag, which means the engine must work harder to move the vehicle. The additional weight of the box and its contents can also contribute to higher fuel consumption. 

The exact difference will depend on several factors, including: 

• The shape and size of the roof box  

• How heavily it is loaded  

• Your driving speed  

• The type of vehicle  

• Weather and road conditions  

• How smoothly you drive  

The effect tends to become more noticeable at higher speeds because aerodynamic resistance increases as you travel faster. 

 

How can you reduce fuel consumption with a roof box? 

Remove unnecessary weight 

A roof box creates extra space, but that doesn’t mean you have to fill every inch of it. Take out anything you’re unlikely to use and avoid carrying heavy items when lighter alternatives are available. 

Drive smoothly 

Rapid acceleration and harsh braking already increase fuel use. With a fully loaded car, the effect can be even greater. Maintain a steady speed, anticipate traffic ahead and avoid unnecessary acceleration. 

Check your tyre pressures 

Underinflated tyres create additional rolling resistance, making the engine work harder. Use the manufacturer’s recommended tyre pressure for the load you’re carrying. 

Remove the roof box when you’re not using it 

An empty roof box still creates drag. If your trip/holiday is over and you don’t plan to use it again soon, removing the box and roof bars can help your car return to its usual fuel efficiency. For more ways to make each tank last longer, read our tips for improving your fuel efficiency. 

 

How should you pack a roof box? 

Before you start packing, check three separate weight limits: 

1. Your car’s maximum permitted roof load  
2. The roof bars’ maximum load  
3. The roof box’s maximum load  

Remember that the vehicle’s roof limit usually includes the combined weight of the roof bars, box and everything inside it. 

For example, if the car’s maximum roof load is 75 kg and the box and bars weigh 20 kg together, you only have 55 kg available for luggage. Always confirm the figures quoted in the manufacturer guide rather than relying on estimates. 

Place heavier luggage in the boot when possible and use the roof box for lighter, bulkier belongings. Spread the contents evenly and use the internal straps to keep items from moving during braking or cornering. 

Once packed: 

• Check that the box is securely attached to the roof bars  

• Make sure every fastening is tightened correctly  

• Secure the contents using the internal straps  

• Confirm that the lid is fully closed and locked  

• Check that nothing obstructs the boot when it opens  

• Recheck the fittings before making the return journey  

Never exceed the recommended roof load, even if there appears to be room for one more bag. 

 

Your pre-journey roof box checklist 

Before pulling away, take a few minutes to check: 

• The roof box and bars are compatible with your vehicle  

• All fittings are correctly tightened  

• The combined load is within the permitted weight limit  

• Luggage is distributed evenly and secured  

• The box is properly closed and locked  

• Your tyres are in good condition  

• Tyre pressures match the manufacturer’s loaded-car recommendation  

• You know the vehicle’s new total height  

• You have checked any manufacturer-specific speed guidance  

 

A final walk around the car can catch a loose fitting, open latch or soft tyre before it becomes a problem on the motorway. 

For extra reassurance before a holiday or long journey, you can also book a free seven-point safety check at Merityre. This includes checks of your tyre condition and pressure, lights, brakes, battery and essential fluid levels. 

 

Prepare your tyres before your next journey 

Before your next holiday or long-distance drive, find your nearest Merityre Specialists branch for a free tyre check. Our team can inspect your tyre pressures, tread depth and overall tyre condition, helping you set off with greater confidence. 

 

FAQs 

Can you drive at 70 mph with a roof box? 

There is no separate UK roof box speed limit, so cars can generally travel at up to 70 mph on a motorway where permitted. However, you must follow any maximum speed recommended by the roof box manufacturer and reduce your speed if the weather, traffic or road conditions require it. 

Do I need to increase my tyre pressure with a roof box? 

You may need to adjust your tyre pressure if the roof box forms part of a heavily loaded vehicle. Check the manufacturer’s recommended pressures for a fully loaded car rather than increasing the pressure by an arbitrary amount. 

What shouldn’t go in a roof box? 

Avoid placing very heavy, fragile or valuable items in a roof box. Heavy belongings are generally better stored low down in the boot, while the roof box should be used for lighter, bulkier luggage. Always follow the box manufacturer’s instructions.