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Monday, September 28, 2015

What readings should I see from my TPMS?

Without knowing the target Cold Inflation Pressure for your personal application, I can only provide some general guidelines to establish the operation of your system.

First off, I recommend that TT with multi axles run a CIP equal to the tire inflation on the side of the tire associated with the max load. This may match the tire placard but you need to read your placard to confirm the numbers.

Motorhomes and other motorized vehicles can use the measured load on the heavier end of each axle to consult published Load & Inflation tables to learn their MINIMUM CIP. For motorized vehicles I suggest a CIP pressure of at least 10% above the minimum number shown in the tables.

Let's assume your CIP is 80 psi, as on most Load Range E tires. One time you need to check your inflation with a good digital gauge and confirm the hand gauge reads 80. Note the tires should be cool and not driven on for at least two hours and not in direct sunlight to get an accurate reading.

Having confirmed the tires are at 80, you can then check your TPMS monitor to see what it is reading. I would not be surprised to see pressure variations of +/- 2 to 4 psi. Remember, TPMS are primarily designed to warn of a loss of pressure, not to be a substitute for hand pressure gauges.

Most TPMS are set to warn when pressure has dropped 15% from the CIP.  I prefer to get a warning before tires get that low so I would set the "base level" of my Tire Traker system at about 107% of the 80 psi on my rear tires to 85 psi. This works out to 72 psi low pressure warning rather than 68 psi low if I went with the original 15% loss number.

Now my TPMS gives both a Rapid and Quick  Early warning when the pressure drops 3 psi from the HOT operating pressure in about 2 to 10 minutes. You will need to learn the details of your early warning settings if you run a different brand TPMS. This early warning is not affected by my CIP or Base Level settings, as it is based on a drop from the actual hot operating pressure.

You can expect to see your HOT pressure to be 10 to 20 psi above your CIP, but this depends on your actual inflation, load, driving speed and road conditions.

You can also expect your operating temperature to be +10 to + 30 above ambient. Since the sensor is external to the tire, it is cooled by the outside air so will never read that actual internal tire temperature. So, in my opinion, if you see +30, you need to slow down and maybe even park for 1/2 hour or so to let the tires cool off. While there are many variables, I would not be surprised to see external TPMS temperature readings to be 10 to 30° cooler that internal tire temperatures so I would consider a +30° reading to be high.

After you drive for a few weeks with your TPMS, you will learn the variation in the readings and should not be concerned about minor variations in the reading.

Hope this helps you understand your TPMS and how to get the information you need from it.

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  1. Good info!

    Question about tire temp. If I have an accurate "infrared" thermometer which I use to check the tire temps, should the sidewalk or contact patch be used? Will either help check internal tire temp?

  2. IR temperature guns may be accurate but in my experience they are not precise enough and they are not measuring the important location.
    You need to read the temperature about 1/8" to 3/6" below the surface of the tire in the location of the thickest part of the shoulder. so neither the sidewall or the tread contact patch are the correct location.
    Rubber, unlike metal is a very poor conductor of heat so your reading may be accurate for average temperature of the 1/2" diameter area the gun is reading but the point of most interest on the surface is in the bottom of a groove which may only be 0.07" wide so your gun can't focus that accurately.

    I suggest you re-read the post


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