In general TPMS is more accurate than uncalibrated pressure gauges. The one advantage of a separate tire pressure gauge isn't so much absolute accuracy, as it should be consistent from tire to tire compared to TPMS where each tire has its own sensor which potentially could measure differently than another. But I have found that TPMS is very accurate and it would be unusual for the TPMS in one tire to be significantly different than another, when both tires are actually at the same pressure as measured by a separate external pressure gauge.

When you consider that your TPMS consists of 4 independent pressure gauges it seems that your TireTek is probably off by 3-4 PSI. Mechanical dial tire pressure gauges may skip one cog/tooth in the mechanism and that would account for being off by 3 psi.

As long as you realize that your TPMS pressure is generally hot tire pressure and not cold tire pressure, it's reliable. If cold placard pressure is 33 psi then by the end of your drive the tires could be anywhere from 35 to 37 psi from heat so please don't lower your tire pressure when it's hot just because TPMS seems higher than the desired cold placard pressure.

I think some tire places might have a calibrated pressure gauge that you can test yours against.

 

Thanks for the reply, I did post a general reply to the replies of my post. I am thinking that the TPMS is probably most accurate from my reading of all the replies. I have a couple questions for you.
I do not understand when you say, "TPMS is generally hot pressure and not cold tire pressure." For example, my TPMS display does not give a readout immediatly on starting the engine and looking at the display. So I will take a ride around the block and relatively quickly get a readout. It is not strictly 'cold' at this point, but it is what is necessary to get the read out. I would think this reading is very close to 'cold' tire pressure vis a vis coming back from a long freeway trip. Are you saying that this is not an accurate TPMS reading relative to a quite hot tire? What I think to myself is that I will consider the 'around the block' reading as cold tire pressure and fill my tires accordingly.
I did watch the video and thought it was informative. Do you know the brand name of the tire pressure gauge the fellow in the video advocated?
Having done few posts on this forum I am finding there is a lot of diversity, and I guess truth in general and particular can be hard to come by.
Thjank you.

 

2020 OB Limited XT. I have 2 separate sets of OE wheels with OE TPMS sensors, one set for summer and the other for winter. All 8 sensors involved read between +0 and -1 compared to a quality digital readout manual gauge spec'ed as accurate to plus or minus 0.2 psi. I can tell from tire rotations that minor differences in dashboard-displayed pressure are associated with a particular sensor, not the position on the vehicle.

Have you cross checked your manual gauge against another quality gauge, for example what a tire shop reads? It seems unlikely to me that all 4 of your sensors would be off by the same wide amount.

 

I have not crossed check the gauge with a tire pressure shop gauge. Perhaps I will do so. It seems in reference to your last sentence as I understand it that the difference would be related to the difference between my tire pressure gauge per se, and the TPMS.
As far as tire pressure shops, I usually go to Discount Tires when I have problems, which is rare. They usually fill the tires to a pressure that seems random to the operator on that particular day, and usually they are high. Say 40 psi by the time I get home. Maybe that is not so far from the 'cold' reading. The Subaru dealers seem to fill to all the tires to 36 psi, which is another thread on the forum. I do not understand why they would not at least maintain the pressure differntial of 2 PSI between the front and back that the tire placard states.
Anyway, I will tend to believe the TPMS and go from there.
Thanks

 

I’ll take my digital tire gauge any day over TPMS.

 

As one who calibrates pressure gages for a living, NEITHER of them are all that accurate.

An analog pressure gage that is calibrated in that size is usually only good to 3% of the range of the gage (We call them "3-2-3" gages as their accuracies are 3% in the lower third, 2% in the middle third, and 3% in the upper third). The digital ones have a higher resolution, but really are not all that much more accurate. The accuracies only hold for a given amount of time and that will vary gage to gage. We work to a 95% probability based on performance of individual gages, and the vast majority only hold their tolerances for 9 months before requiring adjustment.

The TPMS in the Subaru's are direct, which means the sensors are typically strain type pressure gages. While they are generally considered to be accurate to +/- 1 PSI, they aren't ever actually calibrated. The issue with that is drift over time. ALL strain gages drift over time. Many hold within their tolerance between calibrations, but in the cal lab, they are always corrected at each calibration. This is because the drift is usually enough that they would be out of tolerance on a second recalibration without recalculation of the shift.

As was said before, both systems should be used as an indicator of consistency between tires and abnormal drift over time. But, neither are what I would refer to as "accurate" measurements of the actual pressure.

 

@Tray_E What would you recommend for getting calibration checked on an analog gauge? I have an Intercomp 0-60. For the first 3 months I had the car, the TPMS matched it within about 1 psi. Then, on my monthly check, I had to bleed all four, despite checking in the shade at similar ambient. I immediately suspected the gauge. From that point forward, the car has fairly consistently read 2 psi below the gauge.

 

If you are REALLY concerned that the pressure is accurate (within 3%), then there are comercial labs (In the US, Transcat, SIMCO, or just do a search for calibration services). It will run you anywhere between 50 and 100 bucks. Personally, I just keep in mind that it is just a reference value and I use it for consistency.

 

This is not true calibration but worth watching:

 

This is not true calibration but worth watching:

NOOOOooooo!!!!!!
DO NOT DO THE ADJUSTMENT HE POINTS OUT.
That is a Range/Span adjustment. There are three adjustments on a pressure gage that can be made.
1- Range/Span - This increases/decreases range of the gage. If your gage reads 22 at 20, 44 at 40, 66 at 60, then you would decrease the range.

2-Linearity. This is called scale shape error. If your gage reads 22 at 20, 40 at 40, and 62 at 60, then you would correct the linearity.

3- Zero offset. This is just a shift in where the pointer sits on the face (or a digital rezero). THIS IS THE ONLY ADJUSTMENT SOMEONE WHO HAS NO EXPERIENCE SHOULD BE DOING. If your gage reads 22 at 20, 42 at 40, and 62 at 60, then you adjust the zero. If the gage doesn't have a zero setting screw, then the pointer could be pulled off and reset. However, this should NOT be done at zero, as many gages have "Zero Stops" It should be done by putting a known pressure in (other than zero) and setting the pointer at that pressure.

Additionally, I do not know how he "knows" that one gage is "good". Even if he does, that gage is a 3% gage also. So, you could just as easily be setting ALL your other gages wrong. There is a reason we use gages that are at a bare minimum 4 times as accurate as what we are calibrating. (Usually, it is quite a bit more).

 

Considering TPMS was removed on Canadian spec'd. Subaru vehicles due to lawsuits should be a testament to their accuracy. When I called Subaru to enquire why our 2020 and 2022 lack this option, I was told TPMS was designed as a supplement to the standalone guage usage - which is recommended at every fill up, but up here, it appears many folks were using TPMS as the exclusive measure. You can't get a new Subaru in Canada with them. What's odd is that TPMS exists for almost all other brands.

 

Here’s my contribution and I hope it helps at least a little bit.

After putting a couple thousand miles on my Wilderness, I noticed that (both) front and pssngr. rear tires would change pressures almost simultaneously and that the driver side rear tire was a little slow to catch up.

One morning, I set the cold pressures to 35.3 in both front tires, 33.3 in the right rear tire and 33.5 in the left rear tire.

I am using a Jayco digital tire gauge without the little flexible hose.

I took multiple readings before I was satisfied that I had actually set the desired pressure.

I “figured” the .3 bump would account for accuracy errors in the gauge and or the TPMS.

Now the TPMS monitor matches my gauge at least for whole numbers, so I am very happy.

 

I have TPMS (factory sensors on one; aftermarket cloned sensors on another) on two sets of wheels. Both read 3-5 psi below what is indicated by my two external gauges. One of the gauges is a cheap dial I bought at the parts store, the other is a much more expensive, supposedly "calibrated" unit I bought off of Amazon. The external gauges are within 1-2 psi of each other. I tend to believe the gauges for the absolute inflation pressure. But I do use the TPMS to watch for any wild swings that might indicate a punctured tire or something.

 

Thank you all, Being one level above a monkey I read all the replies and tried to synthesize them. in general it seems that Silver Onyx and Ragnar would consider TPMS accurate. Tray qualifies both and suggests either may not be accurate per se. Monsieurmaggot seems to conclude TPMS are not accurate. There we go. I shall have to consult the tire pressure Oracle Of Akron to illuminate the question?

 

And I initially felt pleased that the gauge and TPMS agreed, but now wonder if my gauge has decided to start reading higher. Results of experiments are SO dependent on so many things, not the least of which is the operator.

 

This is the internet - if there is a consensus it's a conspiracy.

 

I'm just saying that placard pressure are meant to be for a car that's (for example) in the shade in the morning after not being driven overnight. To the extent that your tires heat up in the first minute or so that it takes for the TPMS to show up your tires it might be within a PSI of cold so it's not a huge deal, but most people don't look at TPMS the moment it comes on. They look at it mid drive or something. My worry is that some people will get to a gas station after driving and then adjust their tire pressure while the tires are hot, lowering it to cold pressures because the TPMS showed a "too high" air pressure.

Absolute tire pressure is not critical - plus or minus a pound or two is not a big deal and it's more important that the tire pressures be consistent, so in that sense, a separate same gauge used on each tire is still useful even though it may not have absolute accuracy.

People will increase or decrease tire pressure in 3-5 pound increments depending on their needs right? Towing you'll increase your rear tire pressure, in autocross you'll increase tire pressures, in sand you decrease tire pressure. Placard pressures are just a good compromise pressure not the ideal for any given situation.

 

Thank you, What you said makes sense, particularly the last paragraph. I liked your comment re. the Internet.
Did you happen to know the pressure gauge that guy in the video liked, or any analog gauge you could recommend?

 

I'm just getting into the Su-Bar-you fray so take this with a grain of salt or rice whatever floats your boat.

My vintage car has the saem pressure on all tires. Obviously no fancy TPMS here . The one thing I check with my un-calibrated pen gauge is that on a cold start for the week is to ensure the tires are all at the same pressure.. Being uncalibrated the gauge may be biased on either the high side or the low side. For me it does not matter the fact that the tires are all zeroed to the same pressure is what is needed and my gauge is not horrendously out of calibration. The same goes for a newer vehicle in my fleet that does not have a TPMS. This vehicle has a 3 psi difference between the front and the rear placard pressure requirements. I do the same thing, make sure the front tires are at same pressure and using the same gauge make sure the rear tires have the same 3 psi delta.

je

 

Thanks

 

Honestly I don't recommend any particular tire gauge. They are almost all made in china and the medium expensive ones are also made in china probably in the same factory as the ones ten dollars or less, but with fancier fittings and housings. The guts of almost all of these analog dial gauges are the same or similar enough that I don't think there's an appreciable difference. Supposedly electronic gauges are more accurate but I'm old school and want analog. I have a misplaced distrust in cheap digital stuff - yes TPMS is digital but they're not 5 dollar parts.

If you want an accurate guage the NIST ANSI certified good ones are like 50 bucks and even those need to be re-calibrated to ensure continued accuracy so it's somewhat of a crazy chase if you want absolute accuracy.

Myself, I buy a cheap gauge, and as long as it's ballpark accurate compared to other gauges I use it. If it's a deviant compared to my several others then I throw it away. I have no absolute standard to reference but I trust TPMS to be ballpark accurate and it turns out that my tire pressure gauges all pretty much agree with TPMS plus or minus 1 psi - so if my last TPMS (hot) reading is 38 PSI then 37 to 39 is what I expect to see with a pressure gauge. TPMS doesn't show me fractional pressures so when it says 38 I have no idea if it's 37.6 or 38.4.

 

id say TPMS over the gauge- ive used a dial style tire gauge and a pen style gauge and they never read the same psi. i use the dial gauge but set the pressure on the gauge 3 psi higher than i want the TPMS to read, if that makes sense?

 

They say the man with 2 watches never really knows what time it is.

 

From a practical perspective though, the main thing is that the gauge is accurate within the range you're using it for, so if you're measuring 35 psi and it's accurate to within half a pound there, then even if it's off by 2 pounds at 20 psi or 50 psi it doesn't matter that much. Then again if you're an off-roader or a racer and you really do use a wider range of tire pressures you can get a super accurate one and have it re-calibrated periodically.

As a practical matter, TPMS is accurate for all practical purposes (proper normal inflation) unless something is specifically wrong with a defective sensor. Aside from that if you have an actual need for a fully calibrated tire pressure sensor that is accurate to 1% across a certain span/range of pressures. I have found cheap (<$20.00) gauges from sketchy manufacturers that claim Grade A calibration but I'm sure they're lying. Accurate instrumentation doesn't come cheap and it must be re-calibrated periodically.

 

From a practical perspective though, the main thing is that the gauge is accurate within the range you're using it for, so if you're measuring 35 psi and it's accurate to within half a pound there, then even if it's off by 2 pounds at 20 psi or 50 psi it doesn't matter that much.

That is only if you are ONLY using the gage in that very limited range. My car tires run in the 30 - 40 psi range. My bike tires in the 90 - 100 psi range. So, for me, (if I relied on those cheap gages for accuracy) it would matter. Additionally, the half a PSI accuracy you might think you are getting, you are not.... for more reasons than just the accuracy of the gage.

That adjustment he tells you to do can make DRASTIC differences in the range which could actually have you close to the point you set it at and SEVERAL pounds error just five pounds away. I have had the pleasure of having to fix gages that people tried to adjust using you tube videos.

My point was less about getting hung up on the calibration itself and more about letting go of the accuracy you think you are getting from ANY of those gages. You just aren't going to get anything other than something that tells you you are relatively close to what you want to be and consistent between your tires and a little bit of consistency over time (but there WILL be drift).