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I'm really not sure whether the difference is as much as bandied about, particularly with diesel vehicles and modern climate control systems. I have tried hard to measure a difference, and it is hidden by all the other variables.
One problem is that climate control systems are set to a temperature, and so the air-con might well not be engaged a lot of the time. In addition, diesel engines generally have a lot of torque, so the load is not as noticeable as with small petrol engines. Furthermore, with modern HDi engines, there is no fuel consumption when running with no throttle. Yet the aircon will still turn if engaged. Thus, going downhill in gear, the aircon has no effect on fuel economy at all.
Another confounder is that if the external temperature is warmer, the engine will reach its operating temperature a lot sooner, and so run more efficiently. My engine idles at the same speed whether the aircon is on or not. The ambient temperature and the speed you are driving at also has an impact. The aircon also has to dissipate heat under the bonnet. How efficiently it does this will have an effect upon how efficiently it can cool the interior. Travelling at speed will maximise the airflow over the aircon radiator. A lower ambient temperature will increase the differential between the radiator and the airflow, again improving the cooling of the radiator.
Perhaps the best way to determine how much of an impact it has would be to measure the change in 0-60mph times with aircon engaged and off. If it has that much effect, it would change the performance of the vehicle. On an old 1.1l Suzuki Swift, it used to slow down if you were on the motorway and engaged the aircon.
Travelling around in traffic in temperatures of 45°C will result in the aircon having a greater effect on MPG compared with driving around in temperatures of 25° on the motorway.
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