Packing the car: roof box, carriers and securing the load

Packing for a trip in an ordinary car, estate or SUV makes it surprisingly easy to cross limits nobody has looked up. A roof box, carriers, bicycles and camping gear weigh more than they look, change how the car handles and push fuel use up. The binding limits are always those in the owner's manual and the papers of your own vehicle; the figures below come from the sources listed and give an order of magnitude, not a clearance. Rules on carriers and marking differ from country to country.

Payload and roof load: two separate limits

Every vehicle has a maximum permitted mass, called GVWR in the United States. What remains between the empty weight and that limit is the payload, and passengers, luggage, roof box, carrier and accessories all draw on it together. The US road safety agency NHTSA points to the tire and loading information placard, which sits on the door edge, the doorpost, the glove-box door or inside the trunk lid, and to the owner's manual for the maximum recommended load. Its advice is short: do not overload the vehicle. In Europe the values are in the registration papers and the handbook.

The roof load is a separate and much smaller limit. ADAC gives a range of 50 to 100 kilograms for cars, but stresses that the value for your own vehicle is in the owner's manual and that the roof box and its mounting system count towards it. Anyone who fits a box on a carrier has already reduced the usable load before the first bag goes in. ÖAMTC adds that bicycles, heavy e-bikes in particular, must not exceed the permitted roof load, and that roof loads and speed need to be adapted to each other.

Heavy at the bottom, everything secured

Inside the car one simple rule applies: the heaviest items go lowest, so that they affect handling as little as possible and do not slide forward when braking. ÖAMTC advises resting luggage against the backrest, not stacking it above the top edge of the rear seat and keeping the parcel shelf completely clear. Cool boxes and bags belong in the rear footwell, not on the seat, and, as our own addition, the driver's footwell stays empty because anything there can obstruct the pedals.

Figures show why this is taken seriously. According to ÖAMTC, a badly secured 15 kilogram suitcase can become a projectile of about 450 kilograms in an impact at 50 km/h. In an ADAC crash test at 45 km/h, a 145 kilogram load briefly acted on the belts with more than seven tonnes, and unsecured goods partly tore the front seats from their anchors. Secure loads with tie-down straps, lashing straps, cargo nets and anti-slip mats at the existing anchor points. ÖAMTC adds that a safety net between boot and passenger compartment helps if luggage cannot be kept below the top of the seat backs; it must be hooked into proper anchor points. If you fold the rear seat down, ADAC points out that it can also serve as a protective partition.

Roof box: fuel use, speed and height

A roof box costs fuel, even empty. According to ADAC, a car with a roof box uses on average about 18 per cent, or one litre, more per 100 kilometres at 130 km/h. The US Department of Energy gives 2 to 8 per cent for roof racks and cargo boxes in city driving, 6 to 17 per cent on the highway and 10 to 25 per cent at Interstate speeds of 65 to 75 mph (roughly 105 to 120 km/h). The range mainly shows that speed decides. Taking the box off after the trip saves money, and driving slower saves more than any packing order.

ADAC recommends not driving faster than 130 km/h with a roof box; the box maker's own limit may be lower. In crosswinds a loaded vehicle becomes less stable aerodynamically, and ÖAMTC advises adapting driving style and speed to the roof load, especially in bends. The centre of gravity sits higher, and more mass means more energy for the brakes to remove. A larger following distance therefore makes sense, although the sources give no figure for it.

With carrier and box the vehicle is taller than usual. ADAC advises paying attention to the total height before entering car parks, underground garages or car washes. In practice it helps to measure the height once and note it where you can see it, because entrance signs are easily missed on arrival.

At the rear: bike carriers and rear boxes

ÖAMTC regards a tow-bar carrier as the safest option for bicycles. According to the US Department of Energy, rear-mounted solutions also cost less fuel, about 1 to 2 per cent in the city and 1 to 5 per cent on the highway. The tow coupling should be free of grease and paint when the carrier is fitted; as a practical tip of our own, detachable parts and e-bike batteries travel better in the boot. The same should apply to a rear box on the coupling; the permitted nose weight is in the vehicle papers. A trailer is a topic of its own, and there too the papers set the limits.

The carrier must not hide the number plate or the lights. The vehicle lighting must stay visible too, unless the carrier has its own lights. If the carrier hides the plate, ÖAMTC describes two options in Austria: move the plate to the carrier or fit a red supplementary plate. ÖAMTC adds that in Germany the rear plate must stay on the vehicle and may not be moved. As of 2026 and only as examples, so check the rules for each country you drive through. Bicycles must not extend beyond the vehicle width including mirrors. In Italy, according to ÖAMTC, cargo that projects to the rear must carry a warning panel, even on an empty carrier; a metal panel of at least 50 by 50 centimetres with red and white stripes is quoted, and two panels if the load spans the whole vehicle width. The view to the rear also deserves a check, because luggage packed high or a loaded carrier can restrict it.

Before you set off

Preparation includes a tyre check. ÖAMTC advises adjusting the pressure for heavy loads, using the figure in the owner's manual; NHTSA reminds that the stated pressure applies to cold tyres, measured after at least three hours standing. With a heavy load the headlight aim should also be set so that oncoming traffic is not dazzled. Then check roof load, fastening and straps, test the luggage for movement, sort out height and the plate on the carrier, and look again at the first stop on the road.

Sources

• Fahren mit Dachbox (ADAC)
• Crashtest: Darum ist Ladungssicherung so wichtig (ADAC)
• Urlaubsgepäck sicher im Auto transportieren (ÖAMTC)
• Fahrradträger fürs Auto (ÖAMTC)
• Driving more efficiently (US Department of Energy, fueleconomy.gov)
• Tire Safety: Everything Rides on It (NHTSA)