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Friday, July 1, 2022

Why is my TPMS giving different readings than my hand gauge?

 That question was asked:

TPMS vs. Tire pressure gauge. The readings between my TPMS and my quality tire gauge, are always different. On cold days there is even more of a difference. Which one do I rely on?

Here is my answer:

Yes, the inflation reported from TPMS are not identical to the numbers found on your hand gauge. There are a variety of reasons for this.

1. Hand gauges can and do vary, with many off by +/- 1 % or more
2. TPMS are primarily designed to monitor pressure change, not absolute pressure. I have seen statements of +/- 2% accuracy
3. Temperature does affect the tire pressure reading at about a 2% pressure change for each change of 10°F so it doesn't take too long for a tire exposed to direct sunlight in the morning to see a measurable change in pressure.

Stack all of these variables together and I think you can understand why your numbers do not always match.

So "Which gauge should I trust when setting tire pressure?"
My quick and short answer is: Rely on your digital hand gauge when setting tire pressure.

For a little more background I suggest you review the information and numbers in this blog  on the subject. Here are some direct links. Note some of these posts also have additional links with more data, that relate to this basic topic of pressure and gauge vs TPMS accuracy.

TPMS Comparison

How accurate is your TPMS?

As far as I know, I am the only person to do a direct comparison of different TPM systems. As you can see in my posts with data, I found no meaningful difference between the two I tested and I have no reason to suspect any other TPMS to deliver much different results.
As a tire design engineer (ret) I am comfortable with readings that are within a couple of percent (i.e. 2 psi on an 80 psi goal). Part of the reason for this is that I also am a strong advocate for including a margin on the inflation required to support the measured load on your tires. I suggest that if possible a +10% margin on inflation as long as you do not exceed the rating for your wheels. Un-damaged tires can easily tolerate a +10% on the inflation found in the Load & Inflation tables and if you are using bolt-in metal valve stems you have the bases covered.

The practice of checking the tire pressure on the morning of each travel day is a good one and is advocated by many people that are knowledgeable on tires in RV usage. My personal practice is to get up, turn on my TPMS and go make the coffee. By the time I can pour my cup, I can go back and watch my TPMS monitor report the inflation in each tire. I have learned that my system takes about 10 to 15 minutes to get all 6 sensors to report the current pressure. Note this timing is normal. I know this from TESTING my system The low-pressure report only takes a few seconds to register with the monitor. This 10 to 15 minute timing for regular checks, is designed into TPMS to preserve battery power. I advocate that all owners of RV TPMS should test their system annually.

While you did not specifically ask about temperature effects, HERE are a couple of posts from my blog on that topic.

Ya, my wife says if you ask me what time it is I will tell you how a watch is built. That's just the engineer in me.  If you have other questions I believe that you can find the answer in one of my blog posts.

Good Travels.



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