Fuel, speed and noise on a 600 km caravan trip
A family plans a 600 km summer drive from Zagreb to Dalmatia with a 1,400 kg caravan behind a 2.0-litre diesel car. Solo consumption is 6.2 l/100 km; on the previous identical trip with the caravan it was 11.5 l/100 km. They have a roof box on the car, two bicycles on the caravan’s rear rack, and 100 kg of water in the caravan tank. Car tyre pressures are set to the solo figures (2.2 bar front and rear); the caravan tyres are at 2.5 bar although the maximum permitted is 4.5 bar and the handbook recommends 3.5 bar for towing. The driver plans to hold 110–120 km/h on the motorway to get there faster, and to leave the engine idling during a 30-minute ferry queue. The caravan’s MAM is 1,500 kg and the car’s towbar limit is 1,600 kg.
1. Which choices increase fuel use and noise, and which are legally relevant?
Read the worked answer
The planned 110–120 km/h is both a fuel penalty and a legal problem: a car towing a trailer is subject to the lower combination limit, 80 km/h, not the general motorway limit. The roof box and rear-mounted bicycles add aerodynamic drag and can affect stability; the rear rack may also reduce nose weight. The 100 kg of water adds mass that must be accelerated and braked. The caravan tyres at 2.5 bar instead of the recommended 3.5 bar increase rolling resistance and heat, and the car’s solo pressures are not the towing pressures. Idling for 30 minutes wastes fuel and creates noise and exhaust emissions at the ferry port.
The legal points are the speed limit, secure load, correct tyre condition and pressures, and the caravan’s MAM and towbar limits. The environmental points are drag, mass, rolling resistance and unnecessary idling.
2. Propose a concrete plan to cut fuel use and noise without compromising safety.
Read the worked answer
Drain the water tank before departure unless it is needed en route. Remove the roof box if possible; carry the bicycles inside the car or on a properly rated rack that keeps the nose weight within limits. Set the car’s rear tyres to the towing pressure and the caravan tyres to the handbook’s 3.5 bar, never above the 4.5 bar maximum. Plan to cruise at 80–85 km/h, use smooth acceleration, look far ahead and avoid late braking. At the ferry queue, switch the engine off rather than idle.
A drop from about 110 km/h to 80–85 km/h reduces aerodynamic drag substantially, and removing the roof box and water removes mass and drag. The previous 11.5 l/100 km could realistically fall by around 1–2 l/100 km, but the exact figure depends on wind and gradient. The main gains are lower speed, less frontal area and correct tyre pressure.
3. How would you check the caravan’s loading and nose weight before starting, and why does it matter for fuel and stability?
Read the worked answer
Measure the nose weight with the caravan loaded as it will travel. For a 1,400 kg caravan, aim for roughly 5–7%, about 70–100 kg, while staying below the car’s towbar limit of 1,600 kg and the coupling limit. Put heavy items low and over the caravan axle, not at the rear. Check the caravan axle load and the car’s rear axle load, secure the bicycles, and confirm the tyre pressures after loading.
A rear-heavy caravan reduces nose weight and makes the outfit more prone to snaking, which forces the driver to slow down and use more corrective steering. Correct distribution allows stable cruising at the legal speed, so it saves fuel indirectly and reduces tyre scrub and noise. Too much nose weight, on the other hand, overloads the rear axle and lifts the front wheels, so the target is a measured middle range, not the maximum.