I know that it鈥檚 bad for fuel efficiency and the environment if I turn on my car鈥檚 air conditioning on a summer鈥檚 day. But surely it is also bad to drive with the windows open because as my speed increases so does the effective ventilation and the fuel cost caused by added aerodynamic drag. So at what speed is it most environmentally friendly to simply roll up the windows and turn on the air-conditioning?
Duncan Simpson, Milton, Cambridgeshire, UK
