As the owner of a Mk2 or is it 3 as there seems to be some confusion. Whatever it is a 2010 EX I shift. I leave the air con on all the time as seems to be the recommendation. It is set at 18 dropping to 17 if it gets too stuffy.
On a cold morning run it on re circulation for a couple of miles then onto 'Normal' To be honest, I do not find the car sluggish nor poor on acceleration. I was taught the idea is to keep the engine turning at a reasonable speed and neither let it slog nor thrash it.
Friends staying at the moment, they have a 66 plate Hyundai I20. He changes into the highest gear the engine doesn't stall at. He claims he gets 55 to the gallon. I have suggested if he let the engine rev a little he might get even better figures. A couple of days back, he turned the air con as there were four in the car and turned off as quickly as he could. Sorry, there can be no enjoyment in sitting watching the mpg gauge all the time.
After a Rover with straight air con I find the climate control works well and even on mornings were the windows mist, it doesn't take long to clear. My test to see if all is well, just put my hand in front of the vent and if it's cold all is well in my little world. There has been considerable input about climate control on the Jazz an and I understood the consensus is leave the climate control switched on all the time.