What Car? Q&A - How much drag do roof rails cause?

Q: I believe that the roof rails on estate cars reduce the fuel-efficiency of the car, but I cannot find out by how much. Can you help?
Don Hill, via e-mail

A: Putting roof rails on an estate car can increase fuel consumption dramatically, because it adds weight and wind resistance. The exact amount will vary depending on the car and the driving conditions, but survey results suggest that roof rails can reduce fuel-efficiency by around 15%.

Other factors also need to be taken into account when calculating fuel-efficiency, such as whether your air-conditioning is operating, or how accurately your tyres are inflated. Under-inflated tyres increase friction on the road and with it fuel consumption.