Switching gears optimally can help with reduce your car's overall fuel consumption.
Most modern cars come with a shift-indicator, alerting you to when you should switch to a higher or lower gear.
READ: Manual vs Automatic cars: Is it okay to skip gears?
There are a number of factors to consider such as wind, gravitational and mechanical resistance which can determine how much fuel you save per trip. More stress placed on the engine in essence, uses more fuel.
Check out the video below: