car's fuel consumption misconception
There are a lot of conceptions and misconceptions about fuel economy with vehicles that we drive daily. Some of them involve common sense, such as keeping tires on the vehicle inflated to the recommend specification; whereas others are just misconceptions that people accept as the truth.
Air conditioner or windows
The most common question about fuel mileage for drivers today is whether driving with the windows down or driving with the air conditioner will consume less gas? Driving with the vehicle`s windows open (this includes sunroof) will affects the aerodynamic drag while driving the air conditioner on will be additional load on the engine from running the air conditioner compressor. So, we already know that both situations will affect the fuel economy, which one affects it more?
After doing the research and gathering the data, it was concluded that driving with the air conditioner on can cause the vehicle to consume 16% more gas when driving 35mph (60kmh). However, the consumption is reduced at higher speeds because the engine produces more power as rpm increase which reduces the burden on the engine of running accessories such as the air conditioner compressor. So at 55mph (100kmh) the vehicle's fuel consumption is reduced to 9.5% instead of 16% more when the air conditioner is on. Moreover, It is opposite for driving with the windows opened; fuel consumption will increase when driving at higher speeds.
Coasting in neutral
Another thing I always hear people like to do when driving is to shift the gear to neutral into in attempt to save gas. Of course, this is a misconception. Every car built since the 1990s can detect the condition when engine revs are higher than idle with a fully closed throttle. And when the vehicle is under these conditions, all electric signals to the fuel injectors is cut off, therefore, no fuel is injected. In other words, that means if your foot is off the gas while the car's in gear, and the condition is met, you are not using any fuel. This is the same for automatic transmissions and manual transmissions.
In summary, shifting into neutral in an automatic or manual transmission will cancel fuel cutoff. Therefore, it is better to remain in gear and let the drive wheels pull the engine airflow down to where fuel cutoff can be enabled or where fuel flow is minimized.
An addition note is that shifting into neutral takes away the driver's ability to accelerate instantly in a sudden emergency.
This one is obvious. Underinflated tires increase rolling resistance, therefore, requires the engine to work harder in order to propel the vehicle. This will translate into an extra 2% to 3% higher fuel. Also, the extra resistance generates more heat, which can cause the tires to degrade more rapidly.
On the other hand, adding a little more air (max 5psi) in addition to manufacturer's recommended inflation pressure improves fuel economy about 1% to 2% depending on speed. Overinflating the tires makes the vehicle roll more easily, but it will stiffen the ride, decrease handling and braking performance, and will cause more wear on the middle of the tires.
Junk in the trunk
No matter what vehicle you drive or how big the engine is, every little extra weight you add on to your vehicle, whether it was a bag groceries or a passenger, it will still cause the vehicle to consume more fuel. The engine will have to do more work. Every 100 pounds of cargo your vehicle has to carry will cause the vehicle to consume about 2% more fuel.
Roof racks and cargo carrier
Whether you have a cargo carrier, a kayak, a bicycle, or a mattress, any protuberance (changing aerodynamics) on a vehicle's roof top will add to the vehicle's wind resistance. Even will just a roof rack and no cargo can cause the vehicle to consume 2% to 5% more fuel.
Shutting of your vehicle during a stop
Turning off the engine in a conventional, nonhybrid (electrical) vehicle will save gas, but it’s only worth it if you will be stopped for more than 1 minute. If the vehicle is designed to shut off during a stop, then it should be ok. Practically, it is not recommended to do this during every stop light because you will be putting extra wear and tear on your starting system if you turn it on and off too often. Cranking a hot engine to start is among the more stressful things we ask it to do because during cranking, the oil pump generally cannot produce much pressure.
Convertible top up or down during driving on highways
It is very nice and relaxing to cruise with the convertible top down during the hot summer break but is it going to cause you to consume more fuel? Well, at lower speeds (35mph / 60kmh), the added wind resistance from the turbulence created by an open roof is not that great, so there is not much of increase fuel economy with the top down. Wind resistance increases by the square of increasing speed, meaning fuel consumption gets much worse. At speeds 55mph (100kmh) and up can cause the vehicle to consume 3.5% or more. When the convertible top is down, it is possible to get slightly better fuel economy with the windows up. The difference in fuel consumption with windows up or down can range about 0.5% to 1.5%.