The Aerodynamic Cost of Roof Boxes and Bike Racks

The Staycation Penalty

When packing for a family holiday, the boot usually fills up fast, leaving no choice but to bolt a roof box or a set of bicycles to the top of the car. While incredibly practical, these additions completely destroy the sleek aerodynamic profile that automotive engineers spent millions of pounds developing.

The Wall of Wind

At low city speeds, aerodynamics don't matter much. But the moment you merge onto the motorway and hit 70 mph, the physics change dramatically. Aerodynamic drag increases exponentially with speed.

A roof box acts like a parachute strapped to your car. The engine has to work significantly harder just to push that blunt plastic box through the air. According to tests by the Energy Saving Trust, an empty roof box increases fuel consumption by around 14% at 70 mph. If the box is fully loaded and heavy, the penalty can jump to over 20%.

Bicycles: The Ultimate Drag

Bicycles are even worse. With their spokes, pedals, and handlebars jutting out in every direction, a roof-mounted bike creates turbulent, messy air that wrecks fuel economy. A car carrying two bikes on the roof can see a staggering 30% drop in MPG on the motorway.

Mitigating the Damage

If you must carry a roof box, keep your speed down. Driving at 60 mph instead of 70 mph will drastically reduce the aerodynamic drag and win back a large chunk of your lost MPG.

Most importantly: take the box off when you get home! Driving around with an empty roof rack or box for weeks after your holiday is simply throwing money away. Always ensure your holiday travel budget stretches further by pinpointing the best fuel prices along your route before you set off.

Local prices differ far more than most drivers expect. For a worked example, see current petrol prices in Sheffield.

Cheaper fuel beats better mileage

Driving technique is worth a few percent on consumption. Choosing the right forecourt is often worth more. With more than 10p a litre between the cheapest and dearest stations in most towns, a two minute check before you set off usually saves more than a month of careful driving. We publish live prices for 2,283 UK towns - find fuel prices near me or browse prices by region.