Can you drive with a rooftop tent on your car? Fuel, speed and height

Yes. Can you drive with a rooftop tent on your car in Australia: you can, and the law treats the tent as a load rather than a modification, which our state by state guide to rooftop tent legality sets out in full. What changes is everything downstream of that answer. Queensland's transport guidance makes the driver responsible for ensuring the load does not make the vehicle unstable or unsafe and cannot fall or be dislodged, and the same page adds that breaching transport laws can void your vehicle insurance in the event of a crash. Four things change the day the tent goes up there: fuel, the speed you should hold, the height you now have to think about, and how the car behaves when you brake. Here is each one, with the numbers that exist and the ones that do not.

Can you drive with a rooftop tent on your car?

You can, inside three limits that all belong to the vehicle rather than to the tent. The first is the dynamic roof load, which is what the roof carries while the car is moving. The static rating, which is what it holds parked, is a different and much larger number: Rola explains that the static figure is commonly around three times the dynamic one and instructs owners to verify that multiple in the vehicle's own manual, and our guide to dynamic and static roof load works the arithmetic through. The second limit is width. Queensland's rules on projecting loads state that a load must not project more than 150 millimetres beyond either side of the vehicle, which a wide tent on narrow gutters can breach without anyone in the car noticing. The third is security, and it is a habit before it is a number: Rhino-Rack's fitting instructions require every bolted connection to be checked after driving a short distance when the crossbars are first installed, and again at regular intervals.

How much extra fuel does a rooftop tent cost?

Something, and no Australian source publishes a figure for a rooftop tent specifically. What exists is the mechanism and a measured range around it. The Australian Government's Green Vehicle Guide attributes fuel consumption increases of over 20 per cent in some cases to increased air resistance, and names roof racks among the parts to carry on the exterior only when necessary. The measured work is American: Yuche Chen at the National Renewable Energy Laboratory and Alan Meier at Lawrence Berkeley National Laboratory reported a fuel consumption penalty of 0 to 25 per cent on passenger cars depending on the configuration. The bottom of that range is the part worth keeping. A load on a tall 4WD adds less in proportion than the same load on a sedan, because the vehicle was already pushing a large frontal area through the air. Speed then multiplies whatever the penalty is, and the same Green Vehicle Guide puts a car at 110 kilometres per hour using up to 25 per cent more fuel than the same car at 90.

How fast can you drive with a rooftop tent fitted?

No Australian rule sets a speed for a loaded roof, so the answer comes from the rack instead of the road. Rhino-Rack's leg fitting instructions instruct owners to reduce speeds, note that the handling characteristics of the vehicle change when a load is carried on the roof, and ask for extreme caution with wind-resistant loads. Yakima is more permissive and more explicit: its support documentation states that under normal conditions its products are safe to use up to the posted speed limits, with the caveat that strong winds put additional forces on the load and the rack. Only one number is published by any of them and it belongs to a different product, which is exactly why it should be quoted as such: Rhino-Rack lists a maximum speed of 130 kilometres per hour with its MasterFit roof box fitted, a hard shell box, not a folded fabric tent. Off sealed roads the rack derates instead: the same fitting instructions divide the maximum roof load rating by 1.5 for unsealed roads.

What does the extra height actually stop you doing?

Car parks, and it happens well before the legal ceiling. The height limit that applies to a light vehicle on the road sits far above anything a rooftop tent reaches, and our state by state guide covers where that figure comes from. The barrier you meet in real life is commercial. Brisbane City Council publishes a vehicle height limit of 1.93 metres at both its King George Square and Wickham Terrace car parks, and a 4WD sits close to that with nothing on the roof at all. The draft revision of the Australian and New Zealand standard for off-street parking facilities sets a minimum headroom of 2.2 metres to admit cars and most light vans, requires the available clearance to be signposted at entrances where it is under 4.5 metres, and asks for warning devices wherever it falls below 2.5 metres. The number that matters is yours, measured from the road surface to the highest point of the fitted tent, and written somewhere the driver can see it.

How does a roof load change the way the car handles?

It raises the centre of gravity, and every authority that has published on it says the same thing in different words. The Western Australian Department of Transport's vehicle standards bulletin states that carrying any load or equipment on a roof rack raises the vehicle's centre of gravity, which may adversely affect its handling and stability characteristics, and the same bulletin advises that a load with a mass exceeding 50 kilograms should generally not be carried on a roof rack unless the rack manufacturer's tested capacity allows it. The Load Restraint Guide puts the mechanism in one line: the higher the centre of mass of the load sits above the ground, the lower the speed at which the vehicle will overturn when cornering. Rhino-Rack's instructions ask for special consideration when cornering and braking, and for the load's centre of gravity to be kept as low as possible. In practice all of that reduces to one habit: brake earlier than you would empty, and take a roundabout as though something on the roof is still trying to go straight. It is.

Should you take the tent off between trips?

If it is going to sit up there for weeks, yes, and the research behind it points at where the fuel actually goes. The Australian Government's household transport page tells drivers to remove roof racks when not in use, and the study behind that instinct found the waste is not in the camping at all. Chen and Meier reported that vehicles travel four to eight times more distance with unloaded racks than with loaded ones, and estimated that roof racks accounted for 0.8 per cent of United States light-duty vehicle fuel consumption in 2015. So the decision is a frequency question, and the weight is the part you can look up instead of guessing: the Often Shelter inflatable rooftop tent, $1,990 carries a listed weight of 20 kilograms, the Often AirCastle, $1,790 lists 29 and the Often Alligator, $1,990 lists 39. Twenty is a two person lift down from a roof and not a solo one, and that spread is the part you feel every time the tent comes off. Camp monthly and it stays on, and you pay the fuel. Camp twice a year and it comes down between trips. Either way, check the rack bolts on the first drive of every trip, because a tent that lives on the roof is a tent you stop noticing.