An essential element that a car deserves is fuel economy. If the car consumes less fuel and is too effective, it is considered great. On the other hand, there are times when the driver complains about unusually high fuel consumption. Have you ever thought about the reason for the high fuel consumption? If not, let us burst the bubble and know where the problem might be.

1. The defective engine block

First, you should look at the "engine". This block could be adjusted inappropriately, or if you skip
an outstanding service, fuel consumption can suffer. Speaking of gasoline engines, when the replacement of worn spark plugs, air filters or fuel filters is neglected; the fuel consumption would rise.
If the engine happened to be a diesel engine and the injectors or air filters were clogged, fuel consumption would increase exponentially. The two components are essential elements that contribute to the combustion of the fuel, and if they clog, a high fuel consumption would be observed.

2. If you create the wrong gears

If you own a manual transmission car, there is a possibility that you are consuming excessive fuel. This occurs more if you are an amateur rider. Suppose you have engaged the 2nd gear and drive at a speed of 50 km / h. This in turn would require more power to take a step, and thus more fuel economy.
In simple words, if you drive in a smaller gear and the speed requirement is higher, the fuel would be consumed excessively. The key to reducing fuel consumption is synchronizing the speed with gear changes and accelerating to a limit.

3. Driving with the AC "ON"

If you have the habit of driving "ON" with the AC, fuel consumption will increase. Check how often you use the vehicle's air conditioning system. The alternating current is known to increase the load on the engine, which is the reason for a high fuel consumption. The same applies to a car heater! Read the maintenance tips to learn more about the heater and fuel economy concept.
Sometimes the use of AC for fuel consumption can be better. This is when you drive at a speed of 80 km / h or more because the open windows can cause air resistance. It is a good idea to open the windows for a few minutes after driving before turning on the air conditioner. Such a step would cool the car down a bit and thus improve fuel economy.

4. More load in the trunk

Sometimes the fault is not in the mechanical block, but in the load your car carries. If you like to travel long distances, you need to have a lot of things in your car. This may sound comfortable, but it indirectly increases fuel economy. If you want to save fuel, it's best to reduce the extra weight.
You can also remove the roof luggage or the appropriate vehicle if you do not use it frequently. Clean your vehicle by removing unnecessary material to reduce the load to a minimum. Once you're done, you'll notice a change in fuel economy.
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Source:
http://www.eliancars.com/2018/12/four-common-causes-of-high-fuel.html#more

Noted pls. When I buy my car.

From experience I can confirm that a delayed oil change can cause a car to consume more fuel as well.