As with the general discussion, different cars, different results.
Quite clearly the hills discussion, what car, engine, fuel, makes a difference to how to approach hills to get best economy, what works for one may not /probably won't work for someone else.
Same with aircon. My former Astra 1.6 petrol generally took a hit of 5 to 10 mpg when running it, which is around 10 % minimum extra fuel. I guess that' the price you pay for lesser refined , more basic design engine in any car.
That said, I can squeeze 50mpg out of it in summertime...not too shabby but it has it limitations.