Last time I drove more than 1000km over three days with my mother & our required gear my car averaged 49 miles per imperial gallon, thus was in winter with the A/C running about 60% of the time to clear the condensation from inside the windscreen.
I've just filled up after a silar trip with tne same luggage & passenger, this time in temperatures exceeding 40°C, with the A/C running full-time. Fuel consumption; the same: 49 mpg. Not bad at all.