It’s considered a bit of a myth by some, but it is in fact true.
Having windows open can cost you fuel, which pretty much everyone knows by now – but so can air conditioning, which can increase fuel consumption by 8-10%.
If it comes to decision of air con vs open window, it is more economical to open your windows up to speeds of around 55mph as running the a/c at low engine speed creates extra stress and demand resulting in increased fuel consumption. Above 55mph, it’s better to run the air con as opposed to the extra drag caused by open windows.
Every accessory that you use, even the rear screen demister or the radio drains power from the battery, which is charged by the fuel in your engine. Try to only use accessories for the minimum amount of time that you need.
Don’t forget that your car’s air con should be run for at least five minutes every fortnight (even if it’s not needed) to keep the system lubricated and free of leaks, also ensure you use an air conditioning maintenance company to carry out your air conditioning service.