Tire Pressure: Who's Right?

[J0E]

The tire pressure readings on the Jeep’s dash is not accurate, no need to try and source a gauge that will correspond with the Jeep’s tire pressure reading unless your OCD is very high, like CDO high…

Many folks have report they are accurate. Mine are very accurate.

Most gauges are very inaccurate for less than half max. Unless you spend for a really good gauge, get a 0-30PSI gauge for air down.

My Joes Racing 32306 (0-30) PSI Tire Pressure Gauge is within 1 pound from 12-17 PSI. My JACO Elite Digital Low Pressure Tire Gauge - 30 PSI reports the same as the console.

Although Jaco appears to be a small shop that imports all their products from China, they actually dip their toes into this and provide you with a declaration that they test and calibrate every gauge and that they have 100% lifetime warranty on their mechanical gauges. Believe them? Look 'em up. They're a one man little unit b shop in a strip complex.

The rubber grommet fell out of my JACO Elite Digital Low Pressure Tire Gauge - 30 PSI so I sent them an email with a pic. They immediately replied they were sending a new stem. They sent me two new stems, arrived 2 days after the email.

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[smokeythecat]

My experience chasing gauges over the years is that the cheap stick gauges tend to be more accurate than most of the analog dial gauges or the electronic gauges.

How do you know they are more accurate?

 

[Remorseless]

How do you know they are more accurate?

My experience has been that they align more closely with the TPMS readouts. And, odds are, if two measurement devices agree and a third does not, it is more likely that the two are correct and the third is incorrect.

 

[roaniecowpony]

...
The rubber grommet fell out of my JACO Elite Digital Low Pressure Tire Gauge - 30 PSI so I sent them an email with a pic. They immediately replied they were sending a new stem. They sent me two new stems, arrived 2 days after the email.

That's great that they have good CS.

My point about them was that their products are Chinese sourced and their products are the same as many other brands found on Amazon. Their main facility is a tiny office in a commercial strip unit. Their secondary "R&D facility" in Colorado, I couldn't find.

If you go to Amazon and just search for tire pressure gauges, you will get dozens of virtually the same product under different branding. Basically, they select products from a manufacturer's catalog, configure it within the manufacture's offerings, and get their name printed on the dial that makes them different than the others. I believe this business model is a situation where the small company pays for brand labeling and order distribution services from a large manufacturer/importer and the small company does the marketing.

Somewhere there's a large warehouse full of these pressure gauges, marked in all the different brands, and an ordering/distribution services office full of "customer service agents" for all the different contracting businesses. This business model isn't specific to tire pressure gauges, but it is prevalent in off-shore manufacturing, particularly China produced goods.

 

[smokeythecat]

My experience has been that they align more closely with the TPMS readouts. And, odds are, if two measurement devices agree and a third does not, it is more likely that the two are correct and the third is incorrect.

Fair enough, but the general tenor of the comments is that the TPMS isn’t especially accurate.

 

[Remorseless]

Fair enough, but the general tenor of the comments is that the TPMS isn’t especially accurate.

My view on the comments is more that TPMS doesn't jive with the gauges that many have bought, and many folks believe the gauge to be accurate and TPMS to be inaccurate - which is fair enough. My assertion is simply that I've used a whole bunch of gauges - ARB analog, Slime analog, Slime electric, air-compressor-hosted analog, etc - and the only ones in my collection that agree with each other are the TPMS and 3 or 4 stick gauges. Thus, since none of my other gauges agree with each other, or the TPMS, I put some trust in using the TPMS and stick gauges in concert with one another since they coalesce around the same numbers, which gives me more confidence.

For what it's worth, our best legal wheeling spot in NC - Uwharrie - has a wonderful business called Eldorado Outpost that hosts free air for airing up in addition to selling the OHV passes and their tire valve air chucks (when they've not been destroyed) have stick gauges built in, and those also agree with my dinky little stick gauges and TPMS, but not any of the other gauges I've used over the years. So I dunno, it's all anecdotal, but it's been what I've seen and used and how things have shaken out for me. ?

 

[Yankee1019]

Most gauges measure PSIg (pounds per square inch, gauge), which is the difference between the absolute pressure (PSIa) of the tire and the surrounding atmosphere. At 10,000 ft elevation the atmospheric pressure drops to ~10 PSIa from 14.7 PSIa at sea level. So if you take a tire measured at 30 PSIg at sea level, bring it to 10,000 ft without changing its temperature, and measure it again, it will read 34.5 PSIg.

However, in practice, we're unlikely to need to worry about this, because:

1. The effect of tire pressure on tire shape, contact patch, ride, etc. is a function of the gauge pressure, not the absolute pressure, so if you inflate your tire to 30 PSIg at sea level, and inflate it to 30 PSIg at 10,000 feet, it will behave exactly the same. You would just need to let ~4.5 PSI out of your tire to achieve that 30 PSIg once you got to 10,000 feet, but a gauge should be an accurate guide to that, since it measures PSIg, so no big deal.

2. Any gauge you use as a point of comparison would be equally effected, so what agrees at sea level should agree at 10,000 ft.

There is one big exception to this, though: TPMS sensors, since they are entirely contained within the tire, cannot measure a difference to atmospheric pressure, so they have to use an absolute reference, which means they are measuring PSIa and converting to PSIg equivalents using some standard value for atmospheric (likely 14.7). That means that TPMS will be affected by altitude changes, and a gauge that matches the dashboard tire pressure at sea level will likely be under by 4.5 PSIg at 10,000 ft.

Were you using the TPMS readings as your reference when saying it was "off by quite a bit"? If so, I think you're right that the altitude difference is the cause of the discrepancy.

If not, we'd have to look at second order effects not accounted for by this simple model, like temperature differences causing non-linearities in gauge response between the two locations, or non-linearities in the response over different pressures. And nobody besides a gauge OEM is likely to care enough to go down that road.

Having experience with both absolute and relative pressure transducers at work I love this post…. lol

 

[roaniecowpony]

I decided I'd make up a low range gauge. Found a grade 1A (+/- 1%) Ashcroft 2 1/2" 0-30 psi gauge and rounded up the bits.

 

[Ratbert]

I think consistency is more important than accuracy, pick one and stick with it.

Unless it shows 6 psi over reality and you end up having a tire come off its wheel because you thought it was at 16.

 

[roaniecowpony]

If altitude was causing it to be off wouldn't whatever you were using as a reference also be impacted by that altitude?

RB,
He was using the TPMS as the reference. As someone noted earlier in the thread, TPMS is a closed system, which means is must be giving a pressure reading based on absolute pressure, then offset for some chosen pressure altitude, whether sea level or some average that the mfr chose.

Further, tires react to pressure differential between the outside and inside, regardless of absolute pressure. So, as you travel from sea level to Pikes Peak, you tires get harder. If you use a common Bourdon tube pressure gauge (or pretty much any tire gauge), it will measure pressure differential and result in a properly inflated tire.

 

[BDinTX]

If altitude was causing it to be off wouldn't whatever you were using as a reference also be impacted by that altitude?

@phageghost explained it way better than I’d be able to in post 29

https://www.jlwranglerforums.com/forum/threads/tire-pressure-whos-right.97898/post-2026096

 

[Ratbert]

@phageghost explained it way better than I’d be able to in post 29

https://www.jlwranglerforums.com/forum/threads/tire-pressure-whos-right.97898/post-2026096

Yeah, I read his after I posted that. That makes sense.

 

[roaniecowpony]

Alright...."Judgement Day"

I took all my homemade Ashcroft gauges, 0-100, 0-60, 0-30, and a cheapo Amazon "Grit" brand $19.99 stem puller/deflator and went out to the JL to compare them all and the TPMS.

Amazon.com: Grit Performance Tire Pressure Gauge (0-60 PSI) Heavy Duty, with Custom Foam Case, Chrome Caps & Valve Core Repair Tool, Rapid Air Pressure Gauge, Tire Deflators Offroad Accessories, 4x4 Tires : Automotive

If anyone is interested, the test was at about 120ft MSL, Barometric Pressure 29.85, Temperature 79F, Dewpoint 66F, as reported by TOA Torrance Airport, less than a 1/2 mile from the test site.

I used the Ashcroft gauge that was closest to the middle of the range of the gauge to set the pressure. In the 30 psi test, I used the 0-60 gauge to set tire pressure and other gauge readings are shown. You can see TPMS agreed with this gauge in the test conditions. In the 15 psi test, I used the 0-30 gauge to set tire pressure and the other gauge readings are shown.

You be the judge.

 

[roaniecowpony]

Again, tire pressure was set with the 0-30 gauge for this 15 psi test, and the other gauges are shown. I threw in the compressor filler gauge picture (WTF?). Looks like el cheapo was pretty dang good. Should be called "true Grit". ?

 

[Chupacabra]

Alright...."Judgement Day"

I took all my homemade Ashcroft gauges, 0-100, 0-60, 0-30, and a cheapo Amazon "Grit" brand $19.99 stem puller/deflator and went out to the JL to compare them all and the TPMS.

Amazon.com: Grit Performance Tire Pressure Gauge (0-60 PSI) Heavy Duty, with Custom Foam Case, Chrome Caps & Valve Core Repair Tool, Rapid Air Pressure Gauge, Tire Deflators Offroad Accessories, 4x4 Tires : Automotive

If anyone is interested, the test was at about 120ft MSL, Barometric Pressure 29.85, Temperature 79F, Dewpoint 66F, as reported by TOA Torrance Airport, less than a 1/2 mile from the test site.

You be the judge. (Pix from phone coming.)

That's interesting. If I am reading your pictures right, they all seemed to be very close. Like within a half a PSI of each other, even the TPMS (which doesn't do decimals).

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