Roof boxes, weight and aerodynamics
Physics doesn't lie. Drag increases with the square of speed, meaning every item you attach to the outside of your car, and every kilogram you carry in the boot, has a direct and measurable impact on fuel consumption. Some of these impacts are surprisingly large.
The roof box penalty
A roof box is the single biggest aerodynamic penalty most drivers ever add to their car. At 70mph, a typical large roof box increases drag by 30–40%, which translates to 15–20% higher fuel consumption on motorway runs. On a 500-mile motorway journey at 70mph, this can add £15–22 to your fuel cost, often more than you saved buying the box instead of renting one for the week.
Even an empty roof rack (no box) causes a 5–10% drag increase at motorway speeds. If you use a roof rack or bike carrier seasonally, remove it when it's not needed. The aerodynamic benefit of a clean roof pays for itself in fuel within a few hundred miles.
Windows open vs air conditioning
The classic question: is it better to open the windows or run the air conditioning? The answer depends on speed. At urban speeds below roughly 50–55mph, open windows have minimal drag impact and air conditioning draws meaningful engine power (typically 3–5% fuel consumption increase). At motorway speeds above 60mph, open windows create significant turbulence and drag, easily exceeding the fuel cost of running the AC.
The weight effect
Every extra 50kg your car carries increases fuel consumption by approximately 1–2% in mixed driving. This is most significant in stop-start urban driving where the engine repeatedly has to accelerate the car's full mass. On a constant motorway cruise, aerodynamic drag dominates and weight matters less.
- Clear out the boot: a box of tools, old sports gear, or bulky items left permanently in the boot all cost you fuel. Check regularly and remove anything you're not actively using.
- Don't overfill the tank: a full 60-litre tank weighs ~43kg. If you're doing urban driving, filling to half-tank rather than full slightly reduces running weight, a marginal saving but real.
- Tow bar: even without a trailer, a tow bar adds minor drag through its protrusion and disrupts airflow beneath the car. Removable tow bars are worth removing when not in use.
Caravans and trailers: the towing penalty
Towing represents the most severe combined aerodynamic and weight penalty most drivers ever experience. A typical family-sized caravan at motorway speeds can increase fuel consumption by 30–50%, depending on caravan width, shape, and towing speed. The drag penalty increases sharply with speed: towing the same caravan at 55mph rather than 65mph typically uses 15–20% less fuel. The UK's 60mph maximum towing speed on dual carriageways and motorways, often resented by drivers, is not without economic logic.
Caravan width has a disproportionate aerodynamic effect. A caravan wider than the towing vehicle creates turbulence behind the A-pillar that is significantly worse than one that fits within the tow car's width. If you regularly tow and fuel costs matter, a narrower caravan relative to your car is meaningfully better, not just easier to drive.
For trailer use, an empty flatbed or box trailer has less drag than a caravan but the weight effect remains significant in stop-start driving, where the engine must repeatedly accelerate the combined mass. On a steady motorway run, aerodynamics dominate; in urban use, weight is the primary factor. Either way, the extra fuel cost of a towing day is substantial and worth building into journey planning.
If you're loading up for a family holiday with a roof box, full luggage, and four adults, expect 20–25% higher fuel consumption than your normal figure. Plan fuel stops accordingly and budget for it. It's not uncommon for a heavily loaded car with a roof box to use a third more fuel than normal on a motorway run.