Tire Pressure Guide

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 You could be driving on the best quality tires, all brand new but if they are not properly inflated, none of that counts. Improperly inflated tires can be a safety risk for you, passengers in your car and other road users as well. It can also cost you more than necessary due to increased fuel consumption and premature wearing.

There are lots of aspects to think about when it comes to tire pressure.  Here is a comprehensive guide on tire pressures: basic information every driver should have, common challenges and how to deal with them.

How to Check Tire Pressure

Firstly, it must be said that you cannot accurately judge if a tire is properly inflated by looking at it. Use a gauge to get accurate readings.

Unscrew and remove the valve cap from the tire then press the gauge against the valve stem. Press it on such that the hissing sound can’t be heard and the gauge gives you a reading. In a standard gauge, the bar will be pushed out from the bottom. A digital gauge will display the reading on a small screen.

Dangers and Effects of Overinflated Tires

There is common misconception that inflating your tires beyond the recommended psi will increase fuel efficiency and improve handling. It is actually more expensive for you and compromises your safety.

When a tire is overinflated, the tread and sidewall becomes harder and the patch which comes into contact with the road shrinks.

These are some negative effects:

  • Premature wear: The center wears out sooner than the outer edges. When the center is completely worn out you have to replace them even if the edges are still in good condition. Your tires could end up lasting you half the time they would ordinarily last.
  • Loss of traction: Traction loss generally happens when roads are wet. When tires are overinflated, it happens even if the road is dry. This could make you spin out of control.
  • Harsher ride: Overinflated tires give you a bumpier ride than necessary. You feel every bump and dip in the road.

Causes, Dangers and Effects of Underinflated Tires

A tire doesn’t have to be visibly underinflated to be low on pressure. This is why it is important to check tire pressers regularly. Under inflation could be as a result of carelessness when inflating tires.

If you are not clear on what the manufacturer recommendations are, you could leave them underinflated. It could also be caused by a leak which gradually reduces air and leaves you with an underinflated tire.

What happens when a tire is underinflated? When a tire is underinflated, too much of its surface is in contact with road. This causes increased friction which in turn causes premature wear and overheating.

Overheating is dangerous because it could cause tread separation and blowouts. These not only compromise the safety of the driver of the vehicle but also that of other road users.

Negative effects if underinflated tires are:

  • Increased rolling resistance
  • Increased fuel consumption
  • Uneven wear on tires because a larger surface is in contact with the road.
  • Decreased brake efficiency

Maximum Tire Inflation Pressure

There is a specific amount of pressure recommended for every car. This is what experts have found to give the best tire life, gas mileage and handling. For most passenger cars it is somewhere between 32 and 35 psi (pounds per square inch).

You can find this information on a sticker on the door jamb of the driver’s door. Some models place this sticker in the console, the trunk lid or the fuel door. If for some reason it is not in any of these points, check the owner’s manual.

Always follow the psi indicated in the door jamb sticker and the owner’s manual. Don’t follow the pressure imprinted on the tire itself. This indicates the maximum amount of pressure the tire can hold.

What is the difference and why are the two figures important? A standard load tire may have a recommended psi of 35 as indicted in the sticker. A standard load performance and touring tire is designed to take up to 44 psi and this figure is indicated on the tire sidewall. The additional range of pressure (between 35 and 44) is provided to accommodate high speed requirements or other unique handling conditions.

How to Service TPMS

A TPMS (tire pressure monitoring system) is an electronic system which monitors air pressure in the tire. It is designed as a safety measure because it alerts drivers when tires are either overinflated or under inflated. Drivers are alerted via a pictogram display, a gauge or a warning light.

When the light is steady, it means there is something wrong with the tire pressure levels. When the light flashes, it means there is something wrong with the TPMS.

Many repair shops say tire pressure monitoring systems should be serviced when tires are changed or when new tires are installed. The process involves replacing the seal and cap on the stem, the retaining nut and the valve core. After that the system is tested to ensure it is in good working order.

Servicing could also be required when one or some sensors in a direct TPMS has failed or is low enough to warrant replacement.  Lithium ion batteries in TPMS sensors last between 5 and 10 years so if you have had yours for about this long, you are probably due for a replacement.

Special Trailer (ST) Tire pressure

Their name tells it all. These are special tires designed for trailers. They are made using heavier duty material compared to regular passenger vehicle tires with more flexible sidewalls.

The suspension system is stiffer than and not as sophisticated as automotive suspension systems. Their main purpose is to support the vertical load borne by trailers and cornering forces in trailers.

Correct special tire pressure

It is important to keep checking the pressure on your special trailer tires with an air pressure gauge.  Quite often they are below safe air pressure levels when they appear to be just fine.

The correct tire pressure for trailer tires is the maximum psi which is indicated on the tire. Check the tire sidewall for a psi number preceded by a small notation saying ‘max load’.

They are designed to function their entire life with maximum capacity pressure regardless of size. The manufacturer determines maximum capacity which is based on load range, ply rating and tread depth.

Always check tire pressure when the tire is cold, preferably early morning when the vehicle hasn’t been in motion for several hours. Checking pressure after it has been driven for some time is likely to give you an inaccurate reading because driving generates heat. Heat in turn generates pressure which may make it seem overinflated when it is not.

Recommended Winter Tire Pressure

Drivers are always advised to check tire pressure regularly (about once a month). Experts say it is essential to check it even more often when there are drastic changes in atmospheric temperatures such as during winter.

What happens to tire pressure in extreme cold?

When temperatures drop, air pressure drops. Here is the science behind it. When it is cold, molecules in the air move slower and tend to gather together. When atmospheric temperatures rise, molecules move faster and spread out more.

You can use a basketball to test this theory. Leave the ball out in the cold through the night. In the morning you will notice it is slightly deflated. When the afternoon sun comes out, the ball inflates in the heat. The same thig happens inside your tires.

When temperatures drop, tires lose an average of 1 to 2 psi for every 10°F change. If temperatures drop by 20°F, your tires lose about 4psi.

Should you inflate your tires in the winter?

From this explanation, the answer is yes. Experts generally advise increasing psi by between 3 and 4 psi above the recommended level. However, now that you know how it works, you can do the math and figure out exactly how much more air pressure you need.

How much colder is it outside, compared to regular temperatures in the area? You know tires lose about 2 psi for every 10°F. Calculate exactly how much more pressure is needed.

Tire Load Range / Ply Rating

Although it is important for all types of vehicles, it is critically important for trailer drivers to understand their vehicle’s load range and how to identify it.

A tire’s load range, also referred to as ply rating tells you how much weight the tire is designed to carry at the industry recommended pressure. It is given by a group of letters on the tire’s sidewall.

Origin of term ply rating: This is a historical term from the days when bias ply tires were made out of layers upon layers of cotton fabric. The description ‘ply rating’ was used to indicate the number of plies (layers) used. The higher the number, the stronger the tire.

Today, tires are no longer constructed using cotton layers. They are made using a few but equally strong plies. The term ply rating refers to equivalent strengths in comparison to bias ply tires made using cotton layers.

Different load ranges for different vehicles

Tires for passenger vehicles are manufactured in 3 load ranges: standard, light and extra (reinforced).

Markings on the sidewall will tell you where the tire falls. Standard load range will indicate either no letters at all or ‘SL’. For light load you will see the letters ‘LL on the sidewall and for extra load range (reinforced) you should see ‘XL or ‘RF’.

Load ranges for passenger vehicle tires table:

Load ranges – Passenger vehicles
Load ranges Abbreviations Max. load pressure
Light load LL 35 psi
Standard load SL or nothing 35 psi
Extra load XL 41 psi

If yours is a light truck, you can expect to see slightly different markings on the sidewall. For these you will see ‘LT’.  For these load ranges are indicated by B, C, D, E and F all with varying ply rating. Ply ratings range from 4 to 12.

Load ranges for light trucks table:

Load ranges – Light Trucks
Load ranges Ply rating Abbreviation Max. load pressure
B 4 B 35 psi
C 6 C 50 psi
D 8 D 65 psi
E 10 E 80 psi
F 12 F 95 psi

Tire Pressure and Dry Road Performance

When you think about the importance of proper air pressure in your car’s tires, remember that it is not actually the tires which support the weight of the car, it is the air pressure in the tire.

This pressure plays a role in ensuring proper performance in the vehicle like directional stability, driving comfort, response to braking and stable cornering.

Dry road performance: If you are driving on a dry road, like is often the case, make sure your tires are inflated as per the directions indicated in the sticker on the driver’s door jamb. This way the car’s weight is evenly distributed across tire tread patterns thus ensuring maximum stability.

Wet road performance: When driving in the rain, conditions change. Wet roads can cause hydroplaning. This happens when tires lose their grip onto the road due to a layer of water and make the car lift and slide out of control.

Should you make any changes to air pressure on wet roads? Although some experts don’t agree with it, others recommend reducing air pressure when you know you will be driving on wet roads.

The argument here is that slightly deflating your tires increases the size of the contact patch. That is the patch which grips the road so you end up with more grip and more stability.

Possibility of hydroplaning

In an interesting but true contradiction to the idea of reducing air pressure to avoid hydroplaning, comes this. According to an article published by the NHTSA, reduced air pressure only lowers the chances of hydroplaning when the car is driven at fairly low speed, about 50 mph and below. If the car is driven faster, underinflated tires on wet roads becomes even more dangerous than if you had properly inflated tires.

When you get up to 60 mph, the pressure of the water going under the tire increases. An underinflated tire has reduced pressure inside it to push down onto the road.

The result is reduced surface area of the tire coming into contact with the road. As speed increases, say to 75 mph, an underinflated tire loses up to 40% of the surface area in contact with the road.

We can therefore conclude that the higher the speed, (as long as it is above 50 mph) and the less pressure a tire has, the higher the chances of hydroplaning.

NHTSA gives a formula to help you calculate speeds at which you are likely to hydroplane based on the amount of pressure in your tires.

Hydroplaning speed (mph) = 10.35 x √ inflation pressure (psi)

Multiply the square root of the amount of pressure in your tires in psi by 10.35. The answer tells you what your speed limit on a wet road should be. Beyond that you are likely to hydroplane. Here is a table with examples.

Hydroplaning speed table

psi
Square root (√)
Hydroplaning speed
35
5.9
61.0 mph
30
5.48
56.7mph
25
5.0
51.8 mph
20
4.47
46.3 mph

Tire and Loading Information Placard

Vehicles with a gross weight rating (GVWR) of 10,000 pounds (4.5 tons) or under are required to have a placard which gives tire and loading information. Often referred to as a Tire placard, it is usually found on the door jamb at the driver’s door and serves as a reference for users and technicians. It gives details on:

  • The number of people that the car can take and their distribution. That is the number of people in the front and the number in the back.
  • Total weight the vehicle can carry ( total load capacity)
  • Appropriate tire size /Original equipment tire sizes
  • Correct tire inflation pressure for the front, rear and also spare tire.

 The total load capacity is based on the maximum weight the tires, wheels, chassis and suspension can handle. Exceeding the listed maximum would put undue strain on vehicle and risk premature wear or failure of components which could compromise safety.

When modifications which increase permanent weight of the car are made, it is the responsibility of the vehicle converter or dealer selling it to affix a Load Carrying Capacity Modification Label next to the original label. The modification label indicates by how much the modifications have altered the vehicle’s total load capacity.

This is common in food transportation trucks onto which storage shelves are added or trucks with tool boxes.

Applying Tire Load Inflation Tables When Replacing Tires

Tire load and inflation tables are made to assist owners and technicians know the limits when replacing original equipment tires (OE) with optional tire sizes.

When you are dealing with standard size tires, all you need to do to know how much pressure your tires need and what kind of load they can take is check the placard on the driver’s door jamb. Alternatively, check the owner’s manual to get these details.

However, some drivers want to install optional sized tires such as plus size tires. As long as they are within safe, allowable size range, this is perfectly fine. If you chose to do this, you must realize that it changes the maximum load your vehicle can take and the recommended pressure may differ from what the OE tires required. This is where tire load inflation charts come in.

You can get one of these from your local tire dealer or download it from websites of reputable tire manufacturing companies. To be able to understand them and apply the information they contain correctly, you need to know how to read tire size designations imprinted on every tire.

Tire size designation

There are two main types of designations you could come across. The European Metric Tire Size Designation and the P-Metric Tire Size Designation (also known as TRA – Tire and Rim Association) which is valid in North America.

Let’s look closer at P-Metric designations, which would typically appear as such.

P195/65R1589H

Here is what each of these symbols means.

P – Stands for P-metric which is used to refer to passenger car tires.

195 – Refers to the section width in millimeters (mm)

65 – Is the aspect ratio. This is a comparison between the tire’s height and width expressed as a percentage. In this case the height is 65% of tire width.

R – Refers to radial construction.

15 – Diameter of the rim in inches.

89 – Load index or maximum load capacity of the tire.

H – Is the speed rating symbol. It is given as an alphabetical letter and indicates the maximum speed at which a tire can be driven, provided it is in good condition, properly inflated and properly installed.

The following table is an excerpt from a typical load index chart

Maximum load
Load index Kilograms (kg) Pounds (lbs)
85 515 1135
86 530 1168
87 545 1202
88 560 1235
89 580 1279
90 600 1323
91 615 1356

The higher the load index the higher the maximum load.

Speed Ratings

Tire speed ratings or speed symbols give the maximum speed at which the tire can carry the load specified in its load index. This is provided it is in good condition, sufficiently inflated and properly installed.

The following is an excerpt from a speed rating chart.

Speed Symbol Speed
Kilometer per hour (km/hr) Miles per hour (mph)
P 150 94
Q 160 100
R 170 106
S 180 112
T 190 118
U 200 124
H 210 130
V 240 149
W 270 168
Y 300 186

Tire Pressure for Competition Tires

When it comes to racing cars, ideal tire pressure is different from what would be used on an ordinary car.

Why so? On one hand, ensuring your tires have the exact amount of air pressure in a race greatly improves performance, an element which racers are not willing to compromise on. Whatever can be done to increase performance must be done.

On the other hand, we must acknowledge that when a race car starts out, the tires begin at ambient temperature and heat up to an operating pressure or hot pressure.

As the tire gets hotter, the pressure in it increases. Since pressures cannot be adjusted in the middle of a race, drivers must identify appropriate cold pressures which will then be ideal for optimum performance after the tires heat up.

The cold pressure set therefore depends on specific conditions of the area at the time. If ambient temperatures are already high then cold pressures should be set lower in anticipation of drastic increase in temperature.

If ambient temperatures are low and perhaps you are driving on a wet road, cold pressures set may be higher because you don’t anticipate drastic temperature increases.

As a racer, your aim is to identify and maintain an ideal hot pressure for as much of the race as possible no matter how long the race is.

You can use guides such as this one to help you set appropriate cold pressures depending on the weight of your vehicle.

Cold pressure vs target hot pressure guide table:

Vehicle weight
Cold pressure (psi)
Hot pressure (psi)
Less than 800 kg (0.8 tons)
17-22
22-29
0.8 to 1 ton
20-26
24 -32
1 – 1.4 tons
23-27
28-40
1.4 tons and above
27-35
37-40

Conclusion

Tire pressure is one of the most neglected elements of vehicle maintenance. Many people don’t bother to check the condition of their tires until something goes wrong. Make point to check pressure levels in all your tires at least once a month.

Remember that simply glancing at them looking for a visible problem is not enough. Neither is giving each tire a kick every so often. Use a pressure gauge to check each individual tire as well as the spare one.

FAQ

How do underinflated tires affect fuel economy?

Low tire pressure causes an increase in rolling resistance. This means more energy is required to keep the car moving.

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Steven Reilly
Steven Reilly is a qualified mechanic and his passion for cars goes beyond just the technical aspects. He is also an amateur racer and all round car enthusiast. When he is not driving them, he can often be found in his garage under the hood of a rare model. Steven Reilly has lost track of the number of hours he has spent setting up his fine collection of rebuilt models. He believes that cars can provide a constructive and fun opportunity to teach the youth important life skills. In line with this, he is developing a community outreach program, potentially dubbed ‘Cars for change'.

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