100% agreed with Bishal123. Turning the AC off doesn't improve a gas mileage. If you are driving a mid size car like Honda Civic or Toyota Corolla at 55-60 mph with A/C on, the mileage drops just by 1 miles /per gallon which is not a big deal. But it the A/C is turned off and windows are left open, mileage is severly affected as power is wasted to overcome the drag due to speedy air. It's better to get the engine oil changed in time,  tyre pressure checked ( don't let it go below 4 psi from the rated air pressure) and smoothly driven. Cheers.....