Fuel-efficient motorway run with a roof box
You are planning a 420 km motorway journey with a mid-size petrol estate car carrying four adults and luggage. A roof box is fitted, and the rear tyres are at 2.0 bar against the manufacturer's recommendation of 2.4 bar for a full load. The car's trip computer shows an average of 9.8 l/100 km on the last similar trip, when you drove at an indicated 140 km/h where the limit allowed. The route includes a 40 km roadworks section with a 60 km/h average-speed camera zone, and a long climb. You want to cut fuel use without losing much time. The tank holds 60 litres.
1. Which changes give the largest fuel saving for this trip, and roughly why?
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The biggest single lever is speed. Aerodynamic drag rises with the square of speed, so dropping from an indicated 140 km/h to around 110–120 km/h typically cuts motorway fuel use by roughly 15–25% for little added time on a 420 km trip. The second lever is the roof box: it adds drag and, if it is not needed at the destination, removing it can save several percent. Third is tyre pressure - 2.0 bar against 2.4 bar raises rolling resistance and heat; setting 2.4 bar for the full load is both safer and more efficient. Fourth is smooth driving: steady throttle, anticipation, and using engine braking on descents rather than repeated acceleration and braking. The roadworks section enforces 60 km/h anyway, so the average-speed zone costs nothing in efficiency terms if you enter it at the right speed.
2. Estimate the fuel needed and decide whether one tank is enough, stating your assumptions.
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At the old 9.8 l/100 km, 420 km would need about 41 litres. With the changes above - lower speed, correct tyre pressures, smoother driving - a realistic improvement of 15–20% gives roughly 7.8–8.3 l/100 km, i.e. about 33–35 litres. Even allowing for the climb and a loaded car, the 60-litre tank is comfortably sufficient with a reserve. The point of the calculation is not precision but margin: plan to refuel before the tank drops below about a quarter, and note that the climb will raise instantaneous consumption while the descent will partly recover it. Do not rely on the trip computer alone; it can under-read actual consumption.
3. How should you drive the long climb and the following descent for efficiency and safety?
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On the climb, keep a steady speed in a gear that holds it without constant downshifting; avoid accelerating hard to overtake slower vehicles if it means repeated full-throttle bursts. Keep the revs moderate and use the torque rather than high revs. On the descent, lift off early and use engine braking in a lower gear, which cuts fuel to near zero on modern injection engines while controlling speed; do not coast in neutral, which is illegal in practice because you lose engine braking and control. Keep a longer gap because the loaded car needs more distance to stop. Re-check the tyre pressures when cold at the next stop, and confirm the roof box is secure - a loose box at speed is a hazard as well as a drag penalty.