Modern car manufacturers invest millions in wind tunnel engineering to achieve ultra-low drag coefficients (Cd). Adding aftermarket exterior accessories and lugging unnecessary cargo completely undoes this aerodynamic efficiency.
The Massive Drag Penalty of Roof Boxes & Bars
Independent testing conducted by the Energy Saving Trust reveals shocking statistics:
- An empty roof box increases fuel consumption by up to 15% at 50 mph and up to 25% at 70 mph.
- Even empty roof crossbars create significant turbulent air resistance, adding a 5% fuel penalty.
- Rear-mounted bicycle carriers add substantial wake drag when loaded.
Instant Saving:
Always remove roof racks, boxes, and bike racks immediately after returning from trips. Leaving an empty roof box attached for everyday commuting can cost over £150 a year in wasted fuel.
Boot Decluttering: Every 50kg Counts
Every additional 50kg of dead weight carried in your vehicle reduces fuel economy by approximately 1% to 2% (with even higher penalties during stop-start urban driving). Clear out golf clubs, pushchairs, tools, and heavy items when they are not in active use.