The engine will warm up faster under a light load, and the exhaust emission equipment will begin to function sooner. Maintain steady speeds for the best fuel economy. A car uses extra fuel when it accelerates. Be alert for slow-downs and red lights. Travel at moderate speeds on the open road.
Higher speeds require more gasoline use to overcome air resistance.
Remember, however, speeds slower than the flow of traffic can create a traffic hazard. Use the air conditioner only when needed. Air conditioning dramatically reduces fuel economy. Both settings can reduce the air conditioning load - and save gas.