"I feel like my car's fuel economy has gotten worse than before."
"I'm driving the same route, but the gas is running out faster."
"Does fuel economy get worse as a car gets older?"
Have you ever felt this way?
A car's fuel economy isn't determined by the engine alone.
Tires.
Engine combustion state.
Engine oil.
Friction resistance.
Driving style.
Vehicle weight.
Temperature and air conditioning.
Various factors combine to determine the final fuel economy.
Therefore, when fuel economy deteriorates,
Instead of just suspecting "the engine is bad,"
It's important to consider "where is extra energy being used?"
This time, we will explain the 10 most common causes of poor car fuel economy, broken down by perspectives such as tires, engine, friction, and combustion, in an easy-to-understand manner.
What is fuel economy in the first place?
Fuel economy is a numerical value that indicates how far a vehicle can travel with a certain amount of fuel.
In Japan, it is generally expressed as
km/L
For example,
Fuel economy of 20km/L
means that, theoretically, the car can travel 20km on 1L of gasoline.
However, actual fuel economy may not always match the catalog values.
Temperature.
Road conditions.
Traffic jams.
Air conditioning.
Load capacity.
Tires.
Driving style.
and so on.
JAF also explains that actual fuel economy varies depending on weather conditions, operating environment, driving methods, and air conditioner usage.
In other words, fuel economy is not just about
"the performance of the car itself,"
but also about
"the car's condition and how it is used."
Cause 1: Low tire pressure
When fuel economy worsens, the first thing to check is tire pressure.
When tire pressure is low, the amount of tire deformation during driving increases.
This increases "rolling resistance," requiring more energy to move the car forward.
In JAF's user test, fuel economy worsened by an average of 4.6% when tire pressure was 30% lower than the specified pressure, and by an average of 12.3% when it was 60% lower. While these are test results under specific conditions, they clearly show that insufficient tire pressure affects fuel economy.
Even without a puncture, tire air gradually leaks out.
Therefore,
Don't just assume "it looks fine."
It is important to check regularly with a tire pressure gauge.
JAF recommends checking at least once a month.
Cause 2: Frequent rapid acceleration and deceleration
Driving style also significantly affects fuel economy.
Rapid acceleration by pressing the accelerator firmly requires a lot of fuel at that moment.
And if you brake immediately after accelerating, the kinetic energy gained from burning fuel is wasted as heat through the brakes.
In other words,
Rapid acceleration.
Rapid deceleration.
Re-acceleration.
Driving that repeatedly involves these actions does not use energy efficiently.
The Ministry of Land, Infrastructure, Transport and Tourism's "10 Eco-Driving Tips" also recommends driving methods such as:
Gentle acceleration.
Driving with less acceleration and deceleration.
Releasing the accelerator early.
If you want to improve fuel economy,
Maintaining a steady accelerator.
Looking ahead earlier.
Reducing unnecessary acceleration and deceleration.
can make a difference.
Cause 3: Dirty air filter
An engine doesn't run on fuel alone.
It takes in air, mixes it with fuel, and ignites it to generate power.
The air cleaner cleans the air entering the engine.
If the air cleaner accumulates dust and dirt and becomes clogged, it may not be able to supply enough air to the engine.
Honda also explains that if a dirty or clogged air cleaner element is left untreated, it can lead to reduced engine performance and worsening fuel economy.
Especially if,
You've been pressing the accelerator deeper lately.
It's not as powerful at high RPMs.
Fuel economy has also worsened.
In such cases, checking the intake system is also an option.
Cause 4: Deterioration of spark plugs and ignition system
In gasoline engines, spark plugs ignite the air-fuel mixture with a spark to cause combustion.
Spark plugs are consumables.
If the electrodes wear out or their condition deteriorates, stable ignition may not be possible.
If misfires occur,
Worsening fuel economy.
Reduced output.
Vibration.
Incomplete combustion.
can result.
NGK also explains that if misfires occur due to electrode wear on spark plugs, it can lead to worsening fuel economy and reduced engine output.
If fuel economy is poor, and also if you experience symptoms like:
Unstable idling.
Hesitation during acceleration.
Increased vibration.
Then checking the ignition system is an important point.
Cause 5: Engine oil condition or incorrect viscosity
There are many moving parts inside an engine.
Pistons.
Crankshaft.
Camshafts.
Bearings.
Engine oil is used to reduce friction between these parts.
However, oil also has viscosity.
Under conditions where the viscosity is too high, the resistance to moving the oil itself may increase.
Conversely, it is also not appropriate to self-determine and use a viscosity that is too low for that engine.
What is important is not that
"Softer oil gives better fuel economy."
but to use the viscosity and specifications designated by the manufacturer.
As explained in a previous article, 0W-20 and 5W-30 are not simply a matter of one being superior to the other.
Using the appropriate viscosity for the engine design is fundamental.
Also, if the oil level is insufficient, it can lead to internal engine damage, so checking the level is important. JAF also lists checking the engine oil level as a daily inspection item.
Cause 6: Combustion state deviating from its normal condition
Combustion is extremely important when considering fuel economy.
In an engine,
Air intake.
Fuel injection.
Mixing.
Ignition or autoignition.
Combustion.
This series of processes is repeated.
If there is a problem anywhere in this process, it becomes difficult to efficiently extract power from the fuel.
For example,
Injectors.
Spark plugs.
Air flow meters.
O2 sensors.
Various temperature sensors.
The condition of these also relates to combustion control.
The important thing is that it's not a matter of
"The more fuel you put in, the more power you get."
Appropriate air volume.
Appropriate fuel volume.
Appropriate timing.
Only when these are aligned does it lead to efficient combustion.
If fuel economy suddenly worsens or the engine warning light illuminates, it is important to have it inspected before trying fuel additives or other temporary solutions.
Cause 7: Increased friction and drive losses
Not all the power generated by the engine reaches the tires.
Inside the car,
Engine interior.
Transmission.
Differential.
Drive shaft.
Bearings.
Tires.
and so on, friction and resistance occur in many places.
This is the "friction loss" explained in a previous article.
If friction increases, energy is required to overcome that resistance as well.
For example,
Dragging brakes.
Abnormal bearings.
Drive system malfunction.
If any of these occur, the car's rolling resistance may increase.
"The car doesn't roll as far as it used to when I take my foot off the accelerator."
"The car feels heavy."
"Fuel economy also worsened at the same time."
In such cases, instead of just looking at the engine, it's necessary to consider the overall resistance of the car.
Cause 8: Unnecessary luggage and vehicle weight
To accelerate a car, the car body itself must be moved.
Naturally, the heavier an object is, the more energy is required to move it.
For example,
Luggage left in the trunk for a long time.
Tools.
Outdoor gear.
Golf bags.
Unused child care items.
If these accumulate, the weight increases accordingly.
The Ministry of Land, Infrastructure, Transport and Tourism's "10 Eco-Driving Tips" also includes the item "Remove unnecessary luggage."
Furthermore, roof boxes and carriers affect not only weight but also air resistance during driving.
Especially on highways, the impact of air resistance becomes significant.
If you don't need to constantly carry unused items, it's more rational to remove them.
Cause 9: Air conditioning and temperature effects
Fuel economy also varies by season.
Especially in summer, the engine is burdened to operate the air conditioner compressor.
Therefore, heavy use of air conditioning affects fuel consumption.
The Ministry of Land, Infrastructure, Transport and Tourism also lists "Appropriate use of air conditioning" as an eco-driving item.
On the other hand, winter also presents conditions where fuel economy tends to worsen.
Cold engine and oil.
Warm-up required.
Tire pressure also tends to drop due to temperature decrease.
Engine does not warm up sufficiently on short trips.
And other factors combine.
In other words,
"Fuel economy only worsens in summer."
"It only gets worse in winter."
This is not the case.
It is not uncommon for fuel economy to change with the seasons.
Cause 10: Frequent short-distance driving
Shopping in the neighborhood.
Picking up and dropping off children.
Round trip to the station.
Repeated short-distance driving of a few kilometers may lead to poor fuel economy.
This is because the engine is not always operating at its most efficient state immediately after startup.
It takes time to bring a cold engine up to optimal temperature.
Engine oil also tends to have higher viscosity at low temperatures, increasing internal resistance.
Therefore,
Start the engine.
Drive 2-3km.
Stop.
Cool down again.
Restart.
This type of usage tends to result in poorer fuel economy than driving for a long time with a warm engine.
JAF also explains that actual fuel economy is affected by driving conditions from a cold engine state.
When checking your fuel economy display,
It's a good idea to consider "Have I been doing more short-distance driving lately?"
Be careful if your fuel economy suddenly worsens.
The approach differs between a gradual change in fuel economy and
A sudden, significant drop in fuel economy.
For example,
It was 15km/L before, but suddenly it became 10km/L.
The engine warning light came on.
The idling became unstable.
There's a strange smell.
Black or white smoke is coming out.
The car feels noticeably heavier than before.
If you experience such symptoms, maintenance may be required.
Poor fuel economy can be a sign of malfunction in:
Sensors.
Ignition system.
Fuel system.
Brakes.
Tires.
Drive system.
etc.
Don't ignore it just because it's "just fuel economy." If it's accompanied by other abnormalities, it's important to have it inspected by a dealer or repair shop.
Improving fuel economy starts with basic maintenance
When trying to improve fuel economy,
Fuel additives.
Oil additives.
Various tuning parts.
It's easy to focus on these.
However, there are basic things that need to be checked first.
Is the tire pressure correct?
Is the engine oil appropriate?
Is the air filter clean?
Is the ignition system working properly?
Are there any warning lights on?
Are the brakes dragging?
Are you carrying unnecessary luggage?
First, ensure the car is in normal condition.
This is the most important thing to consider for fuel economy.
Can fuel additives alone improve fuel economy?
Fuel additives serve various purposes.
Cleaning the fuel system.
Approaching fuel characteristics such as cetane number.
Fuel stability.
Approaching combustion.
and so on.
Therefore, adding a fuel additive will not solve all fuel economy deterioration.
For example,
Low tire pressure.
Dragging brakes.
Faulty spark plugs.
Clogged air filter.
In such situations, the cause itself needs to be rectified.
Fuel additives are not
a substitute for basic maintenance.
It is important to use them for their intended purpose after confirming that the vehicle is in normal condition.
Next Science's view on "fuel economy and energy loss"
When considering fuel economy,
Just looking at "how to burn fuel" is not enough.
The energy contained in fuel, after going through
Combustion.
Heat.
Engine internal friction.
Oil viscosity resistance.
Transmission.
Drivetrain.
Tire rolling resistance.
becomes the power that ultimately moves the car forward.
During this process, a lot of energy is lost.
At Next Science, we consider the condition of vehicles and machinery from multiple perspectives, including:
Combustion/Fuel.
Friction/Lubrication.
Electronic environment.
What is important is not simply "reducing fuel."
Where the generated energy is used.
Where it is lost.
How it reaches the tires.
It's about seeing the overall energy flow of the car.
5 items to check first to improve fuel economy
If you're unsure where to start, these five items are a good place to begin.
-
Adjust tire pressure to the specified value.
-
Remove unnecessary luggage.
-
Reduce rapid acceleration and deceleration.
-
Check the status of engine oil and consumable parts replacement.
-
Check if your recent driving conditions have changed.
These five things can be checked without major modifications.
And if fuel economy is still noticeably worse than before, then checking the
Ignition.
Intake.
Fuel.
Drivetrain.
Brakes.
can help narrow down the cause.
Summary: The cause of poor fuel economy is not just one thing.
There is not just one cause for poor car fuel economy.
Tire pressure.
Driving style.
Air filter.
Spark plugs.
Engine oil.
Combustion state.
Friction/drive loss.
Vehicle weight.
Air conditioning and temperature.
Short-distance driving.
Various factors are involved.
That's why,
"Additives because fuel economy is bad."
"Change oil because fuel economy is bad."
Instead of just trying one method,
It is important to first isolate the cause.
A car converts the energy of fuel into power in the engine and transmits it to the tires through many parts.
How much energy is lost along the way?
And how efficiently is it used for driving?
By having this perspective,
"Why has fuel economy gotten worse?"
will become clearer.