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technically using the air con will use more fuel as its extra strain on the engine to run it - therefore using more fuel.
Driving with windows down will also use more fuel as it affects the aerodynamics of the vehicle but only noticable really at high speeds - the Clio is pretty sh*t aerodynamic brick so I wouldnt worry too much about it.
In my old Pug 306 D, if you put the aircon on when trickling along on tickover in traffic, it was like youd just stabbed at the brakes. Then when the engine woke up and realised it was loaded by the compressor, it would fuel more to compensate.
Made me realise just how much of a load on an engine an aircon compressor actually is.
Yes A/C does use a little more fuel, and yes older vehicles used even more.. Compressors now are pretty efficient and it is not that noticable..
Not running the A/C regularly can risk the Pipe seals drying out and leaking or at worse, causing the compressor to be starved of oil and therefore running the risk of it seizing. So it should be run at minimum once a week for 20mins or so.. Not running the A/C also allows bacteria growth on the Evaporator, that is why you get that "smelly Sock" odour if it hasnt been used for a while.
Remember also that A/C is not only useful during the hot months.. It acts as a dehumidifier, and that is why in winter when the A/C is switched on, the inside of the windscreen clears of condensation/moisture very quickly..
In reality you should run it all the time.. 1: to protect the working parts. 2: to keep you cool and moisture free..