Yes using cruise control will improve your MPG. Drive your car at 40mph in 5th gear, then switch on cruise control. you will see a drop in the revs. Less revs = better MPG. When in motion all of the cars energy comes from the engine. It is not just air con that effects MPG but anything else you switch on when driving. The alternator and air con compressor take energy from the engine so the engine has to work harder to maintain the cars speed and power lights, radio, windows, air con and anything else I have not thought of. Just running the engine and nothing else is how the official consumption figures are achieved.