What That Roof Rack Is Really Costing You

Road trips encourage a simple packing strategy: when the cabin is full, put the rest on top.

That extra space is useful. Aerodynamically, however, the roof is expensive real estate.

Automakers shape mirrors, glass, body panels and even underbody components around airflow. A rooftop box, luggage or bicycles can disrupt that carefully managed air and increase drag. At highway speed, the engine must continuously supply the energy needed to overcome it.

Drag grows in importance with speed

A roof carrier may seem insignificant while moving through a neighborhood. On an interstate, the air itself becomes a much larger part of the vehicle's workload.

This is why exterior cargo and unnecessary high speed can compound each other: the object increases aerodynamic drag precisely when aerodynamic forces matter most.

Weight and drag are different problems

Extra weight does matter, especially during acceleration and in stop-and-go driving. But a rooftop carrier introduces something additional: it changes the shape being pushed through the air for every highway mile.

That makes an empty carrier worth thinking about too. Once the trip is over, a removable roof box that stays attached for months can keep imposing an aerodynamic penalty even though it is no longer carrying anything.

Pack strategically

When practical and safe, use interior cargo space first. If exterior storage is necessary, follow the vehicle and carrier manufacturers' load, mounting and speed instructions.

Rear-mounted cargo can sometimes disturb airflow less than rooftop cargo, but compatibility, visibility, lighting, license-plate clearance, tongue weight and safe attachment all matter. Fuel economy should never override safe loading.

Think beyond one tank

On a 500-mile trip, even a modest percentage change in efficiency has far more opportunity to accumulate than it does on a two-mile errand.

That is what makes aerodynamics particularly relevant to road-trip planning: the vehicle spends a large share of the trip at the speeds where drag matters most.