What is proper tire pressure? Is it not necessary to increase air pressure on expressways? |MOBY [モビー]

Why you need to check tire pressure


Tire air is slowly leaking out.

A survey conducted by the Japan Automobile Tire Manufacturers Association (JATMA) found that the air pressure in passenger car tires naturally decreases by about 5% per month.

What happens if the tire pressure is not correct?

Insufficient air pressure can cause tires to burst

If the tire pressure is insufficient, the contact patch will collapse and load will be applied to both ends of the tire.

This is very dangerous as the cord inside the tire may break and the tire may burst when you run over a curb. Other symptoms that may occur include:

  • Reduced tire life due to premature tire wear
  • Uneven wear occurs in which the tire wears unevenly
  • Tires don’t roll well, resulting in poor fuel economy
  • Deflection increases and steering stability deteriorates, especially when driving on highways
  • lead to tire burst
  • It is difficult to discharge water accumulated on the road surface, making it easier to cause hydroplaning.
  • The original grip power of the tire cannot be demonstrated, and it becomes easy to slip.
  • Harder to stop suddenly during sudden braking

Tires wear out easily even if they are over-inflated

Just as it’s okay for the air pressure to drop naturally, it’s also not good for the tire to over-adjust.

Excessive air pressure causes the center of the tire to swell, reducing the contact patch. This makes it easier to wear out, and the following symptoms may occur, so be careful.

  • The central part of the tire wears out, causing uneven wear and shortening the life of the tire.
  • The tread surface of the tire becomes easily damaged
  • Vibration is not easily absorbed, making the ride uncomfortable

For long and safe use of tires

Under-inflated or over-inflated tires can cause all sorts of problems.

In order to use it safely for a long time, check the air pressure once a month as a guideline.

It is also recommended to have an air gauge that can measure the air pressure so that you can check the tire pressure yourself.

Click here for more information on tire burst

What happens if you don’t have the correct air pressure in your tires?

©Takuya Toga/stock.adobe.com

There is a proper value for tire pressure. The reason why there is a proper value is that if the air pressure is too high or too low, it will cause trouble.

lower fuel consumption

If your tires are not properly inflated, your fuel economy will suffer. The reason is that rolling resistance increases.

Rolling resistance refers to the force of friction with the ground when a tire rolls. Friction increases or decreases in proportion to the contact area between the tire and the ground.

In other words, the lower the air pressure and the greater the deflection of the tire, the greater the contact area with the ground, and at the same timeFuel consumption worsens due to increased frictionIt is.

moreoverSteering is heavy when air pressure is extremely lowThere are also abnormal symptoms that people can feel. Therefore, it is important to check the air pressure about once a month and replenish it if it is insufficient.

shortens tire life

Too high or too low air pressure can cause abnormal wear.As a result, the life of the tire is greatly shortened.

The reason why it causes abnormal wear is because the contact area with the ground changes as I told you earlier.

If the air pressure is too high, only the center of the tire will hit the ground, and the tread in the middle will be too small, even though there is plenty of tread on the sides.

On the other hand, if the air pressure is too low, the tire will sag, putting more load on the sides than necessary and reducing the tread from the sides. In other words, if the air pressure is not properly maintained, partial wear will be severe. As a result, the tread of the tire disappears, shortening the usable period.

In additionThere are many problems caused by not following the air pressure, such as cracking and being weak against shocks.In order to use the tire for a long time, it is necessary to check the air pressure appropriately.

risk of bursting

If the tire pressure is too low, it will cause a burst.A burst is when the tire explodes.

Low tire pressure causes standing waves. This phenomenon is caused by uneven pressure in the tire.High and low air pressure areas occur inside the tire., the surface becomes wavy.andIt will eventually burst.

What happens if a tire bursts while driving? Suddenly, the steering wheel is taken and the car exceeding 1 ton can not be steered. If there are people or cars around, you can predict that it will become a big accident.

The most frightening aspect of this phenomenon is that even if the standing wave phenomenon occurs while driving, the driver will not notice it.

Bursts are common on highwaysIt is one of the bulls, and it is a trouble that has a high possibility of causing an accident. Let’s do daily maintenance firmly for safety.

It is difficult to judge the air pressure of tires that have been increased or decreased by an inch.


Inch up is to change the outer diameter (inch) of the car wheel to a larger diameter (inch) than the genuine one. Inch down is the opposite of inch up, it is to change to a smaller one than genuine.

For basic inch-up and inch-down, it is recommended that the outer diameter be as close as possible to the genuine tire. So when you look at the tire from the side, it will be thinner if the wheel is larger than what you were used to, and thicker if it is smaller.

When you go inch up or inch down, the conditions will be different from those of genuine tires, so it will be difficult to judge the air pressure.

If the tires are not inflated to the correct air pressure, fuel consumption may deteriorate and abnormal wear may shorten the life of the tires or cause them to burst, so be careful.

Maintain proper air pressure for tires that have been raised or lowered by an inch. If maintenance by yourself is difficult, we recommend that you consult with a specialty store instead of overdoing it.

How to check the proper air pressure for tires that have been raised or lowered

Here’s how to find out.

  1. Check the load capacity value of genuine tires at proper air pressure
  2. Check the load capacity correspondence table in the new tire catalog to see what fits
  3. Find the proper air pressure according to the tire standard

I will explain step by step.

1. Check the load capacity value of genuine tires at proper air pressure

When looking for the proper air pressure for tires that have been raised or lowered by an inch, first check the genuine load index (LI) and the load capacity value at the proper air pressure.

For example, if the listed tire size is “205/55R16/91V”, the genuine load index will be “91”.

If you look at the “LI91” column in the tire load index correspondence table, the load capacity value for each air pressure is listed next to the tire size.

2. Find out what fits in the load capacity correspondence table in the new tire catalog

There are two points to note here:

  • The load index should be around the genuine value
  • Ensure that the numerical value of the load capacity is higher than that of the genuine product.

While suppressing the above points, let’s find out what fits in the load capacity correspondence table of the new tire catalog.

Even if the aspect ratio of the tire changes, the proper air pressure basically does not change, so the proper air pressure here is the same value as the genuine proper air pressure.

3. Find the proper air pressure according to the tire standard

Based on the values ​​obtained so far, determine the appropriate air pressure for tires that have been increased/decreased by an inch.

There are three tire standards: the JATMA standard, which is the Japanese tire standard, and the ETRTO standard, which is the European tire standard, and the extra load (reinforced) standard.

The maximum air pressure for each is as follows.

  • JATMA standard: Maximum 240kPa
  • ETRTO standard: Max. 250kPa
  • Extra Load (Reinforced) Standard: Maximum 290kPa

Let’s make sure that it is an appropriate value from the genuine air pressure, the load in the tire standard, and the air pressure at the load load.

How to check the correct tire pressure

Pneumatic Seals for Subaru Legacy
Pneumatic Seals for Subaru Legacy

You can check the proper value of air pressure in the air pressure check table.The air pressure checklist is affixed to the driver’s seat pillar (the pillar that separates the driver’s seat and the passenger’s seat).

The instrument for measuring tire pressure is called an “air gauge”, and it is measured from the air valve that inflates the tire.

The unit of air pressure is “kPa (kilopascal)”. In the past, kgf/㎠ (kilogram force) was used, but with the movement to standardize all units around the world, kPa has become the mainstream unit of air pressure in Japan.

By the way, 1kgf/㎠≒about 98kPa, so you should remember that “2.4kgf/㎠≒240kPa”.

The appropriate value of air pressure varies depending on the vehicle type.

The proper tire pressure varies depending on the vehicle type. Also, be aware that even the same car model may have different tire sizes. for that reason,Always check the air pressure checklist when refilling by yourself.

As an aside, in the case of heavy-duty tires such as off-road tires, the air pressure may be set higher. In other words, even if the proper value for the car is 240kPa, the proper value for the tires may be 300kPa.

In that case, there is no problem even if it is adjusted to the appropriate value on the tire side.

However, it is different depending on the tire and car, such as freight vehicles, etc., and it may be set low if the tire size is large, so let’s check the appropriate value once.

Basically, the same air pressure is OK on general roads and highways.

There is a description on the sticker that “the tire pressure is the same for normal/high speed driving”, so there is usually no need to change the air pressure for general roads and highways. More on that later.

Of course, as an exception, for tires with a low aspect ratio, that is, tires with a thin tire thickness, which are doing things such as increasing the inch of the wheel in order to dress up and improve maneuverability, we dare to raise the air pressure of the studless tires by about 20 to 30%. There is

By increasing the air pressure, it is done to reduce the deflection of the tire rubber and reduce the vibration when the wheel and the ground come into contact.

How to check tire pressure

Check yourself with an air gauge

To check, simply remove the air valve cap and insert the air gauge nozzle. Then the air gauge needle will move. Check the air pressure when you stop.

As a point of caution,Apply a certain amount of force to hold down the nozzlething. If the pressure is too light, the air will continue to escape.

Unless you have a compressor or an air pump with a tank, it’s difficult to inflate your car’s tires yourself, so be careful when pressing down.

Ask a shop to check the air pressure

If you don’t have a special tool, you can ask a shop like the one below to check it for you.

  • gas station
  • car shop
  • private repair shop
  • dealer
  • tire dealer

Most shops that sell cars will check it for you.Above allThe most reliable store is a gas stationI guess.

Gas stations often rent air pumps with tanks so that customers can refill and check their own gas.

In addition, dealers and car shops can also ask you to check when you are asking for work such as an oil change.

No more need for “Increase air pressure on highways”?

© jaimax/stock.adobe.com

There is a legend that has been passed down for decades that it is better to increase the air pressure, especially when driving on highways.

When the tire rotates at high speed, centrifugal force is applied and it is crushed. The more heat it gets, the more likely it is to explode.

Back in the days when tires were less durable than they are today, this kind of phenomenon was especially likely to occur if the air pressure was low, and it was said to increase the air pressure before driving on the highway.

However, I think that there is no problem if you consider this as a story of the past.Tires are now more durable, so you don’t have to worry about changing them depending on the road you run on.If the air pressure is appropriate, it is fine on general roads and highways.

Also, regarding fuel consumption, if you are aiming for the numerical value announced by the manufacturer, it is best to drive with the air pressure of the standard value.

If you really want to set the air pressure higher, keep it at around 10%.

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