Air Pressure, Vehicle Load, and Tire Pressure Monitor Systems (TPMS)

Todays tires are marvels of engineering, but when run at low air pressure they are prone to blow outs at high speed, especially when overloaded or in hot weather conditions. To prevent blowouts, maximize your fuel economy, and extend the life of your tire, we recommend checking the air pressure in your tires at least once a month. We will happily do this for free at any of our locations. Just stop in and we will fill your tires up to the proper pressure with regular air, or we can refill your tire with Nitrogen for a nominal fee.

What is the Proper Air Pressure for my Tires?

Pnuematic Tires are engineered to give optimum performance when properly inflated. The maximum load carrying capacity and inflation pressure that a tire is engineered to withstand is moulded into the sidewall of every tire and typically looks like this…


But the maximum tire inflation pressure is not the amount of air your should put in your tires. Instead, you should look at the drivers side door jam for the Vehicle Placard. It will look something like this:


The Vehicle Placard shows the recommended “cold tire pressure” for your tires as the engineers of your vehicle intended, so that the tires will have the ability to carry the car’s weight (including passengers and cargo), at the maximum speed that the tire is designed to withstand, while also maximizing the tires handling and fuel economy characteristics.

Note that the tire pressure is intended to be measured when the tire is cold. Some of you may remember learning about Boyle’s Ideal Gas Law in your high school physics class. Well, PV=NRT is still the law today! The “P” is for Pressure and the “T” is for Temperature, so if you measure the air pressure in your tires after they have heated up from driving, it will be higher than if the measurement were taken when the vehicle has been sitting for a while. So it is best to measure your tires air pressure in the morning before you have driven a significant distance.

Air Pressure and Vehicle Load

The weight of your vehicle can usually be found on another door jam sticker that looks something like this:


In this case the Gross Vehicle Weight Rating (GVWR) is 3,828 pounds. According to the Vehicle Tire Placard, the maximum additional weight of the passengers and cargo that this Volkswagen Beetle can carry is 772 pounds, for a total of 4,500 pounds, or 1,125 pounds per tire. Note that this is less than the maximum load of the tire shown in the first picture (1356 pounds). This is because the load carrying capacity of the tire will go up or down as the air pressure varies inside the tire. Max pressure means max load carrying capacity, but for this Beetle the engineers at Volkswagen have recommended a lower tire pressure to improve other tire attributes such as handling, ride comfort and traction. If you need to carry more than 772 pounds, you can do so by inflating the tires over the recommended psi, but don’t exceed the maximum shown on the sidewall. By overinflating your tires, you may adversely affect other characteristics of the tires performance.

The tire inflation pressure shown on the Vehicle Placard is recommended by the engineers who designed your vehicle to give the best combination of ride comfort, handling properties, load carrying capacity, and rolling resistance. But you may decide to vary your air pressure to increase or decrease various vehicle characteristics affected by your tires. As a general rule:

Reducing a tires inflation will:

Increasing a tires inflation will:

One other consideration, during the winter tires are prone to lose air quickly, especially when a cold front blows through, so we recommend running your tires with 3 to 5 additional pounds of air to prevent blow outs during Chicago’s typical winter weather.

What is TPMS?

Since September 2007, the United States Department of Transportation (DOT) National Highway Traffic Safety Administration has adopted a Federal Motor Vehicle Safety Standard that requires the installation of tire pressure monitoring systems (TPMS) that alert drivers when one or more tires become significantly under-inflated. The safety standard applies vehicles with a gross vehicle weight rating of 10,000 pounds or less. Many vehicles produced earlier have TPMS also. This federal mandated safety standard is of the same sort that requires seat belts, and from a legal standpoint, by-passing the TPMS system would be the equivalent of cutting your seat belts.

How Do Tire Pressure Monitoring Systems Work?

In most cars and light trucks, the mandated Tire Pressure Monitoring System comes in the form of an electronic sensor attached to the valve stem. The electronic sensor is battery powered and transmits the sensor ID and the tires inflation to a receiving antenna in the car. When the air pressure inside any of the tires drops 25% below the recommended cold tire inflation shown on the Vehicle Placard, a warning light on your dashboard will go on. The warning light will look like the figure on the right if a tire is underinflated, and the figure on the left if the system is malfunctioning.


TPMS Basics

To better understand your TPMS system, please watch this short video that explains TPMS in laymans terms:

Watch Here

Common TPMS Problems

When congress specified that car manufacturers were required to implement a low tire warning system, they didn’t specify exactly how it should be done. As a result, engineers at each car manufacturer came up with a wide range of warning systems. Some were pretty good. Others not so much. Over the last decade the problematic warning systems have been weeded out of the marketplace, but there are still a lot of cars on the road that have cranky TPMS systems and sensors. Here are a few of the common issues and concerns that drivers should be aware of regarding TPMS:

Another point of interest, each TPMS sensor transmits a unique identifying number via radio waves to the cars reciever. Some folks say this federally mandated vehicle component creates a privacy concern that could allow Big Brother to track our movements. Personally I think they give our government too much credit, but to allay the fears of the lunatic fringe, we put in new TPMS sensors every day and do not report the transmitter information to Big Brother.

TPMS Servicing Tips

Each TPMS Sensor is made up of several components. The components for the metal valve style TPMS sensors typically look like the next picture. This is the cranky style of TPMS valves. Tire dealers worldwide detest the engineers who designed this style of TPMS sensor:


The components for the rubber valve style TPMS sensors are shown in the next picture. This type of TPMS sensor is much easier to service and seems to be taking over the industry. If you have to replace a TPMS sensor, go with this style if you can:


We normally charge aroud ten bucks for these components and the labor to install them, and the standards of our industry say that the components should be replaced each time the tire is serviced to minimize the common TPMS problems discussed above.

But the reality is that replacing all these components sometimes creates more problems than it solves, as disassembly of the TPMS Sensor can sometimes end up damaging the valve, requiring it’s replacement. So in practice we allow our technicians some leeway in this matter. If in our technicians judgement disassembly of the valve will cause irrepairable damage to the sensor, we will give you the option of not replacing these components, with the understanding that the original components may fail at a later date allowing air to escape from the tire.

TPMS Relearn Proceedures

On many of todays cars, the TPMS sensor transmits a unique identifier code which is mapped to the wheel location to which the TPMS Sensor is attached. For these vehicles, the dashboard will display which tire is underinflated. When a TPMS sensor is moved around on these types of vehicles when we rotate the tires, it may be necessary to help the vehicle “relearn” where the sensor has been moved to so that it will properly indicate which tire is underinflated. The relearn process also has to be done when a new sensor is installed onto the vehicle. This process is another part of the TPMS world that is simple on some cars and challenging on others. So if you have one of those “special” cars, please have patience with us. On some vehicles, the new TPMS sensor will awaken itself once the vehicle has been put into motion and it will sync up with the transmitter on its own after a bit of driving. Other vehicles require a technician to spend around 15 minutes using a special TPMS programming scan tool to reset the light. Some DIY folks have asked about buying their own TPMS programming scan tool, but when told that the tool runs $1200 on the low end, they happily pay us the $10 fee we charge for reprogramming the system and turning off the TPMS light on the dash.