Tire Pressure: Who's Right?

[Gorilla57]

I have one of the Currie rapid deflate setups and that digital gauge is off and reads low by 4-5psi. I have analog gauges and stick ones that match the TPMS exactly. So, when I use the Currie, I just let out the extra 5psi and go. Digital are supposed to be very accurate, but they seem to be the worst offenders out there for being off.

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[jbcrane]

Thanks everyone for chiming in. You've confirmed my suspicion: there is no empirical standard and as long as all tires are the same - and within a few psi of the desired target, I'm just gonna roll with it. Pun intended.

 

[Tech Tim]

There's an old adage.
A person with a tire pressure gauge always knows their tire pressure.
A person with two or more tire pressure gauges is never quite sure.

That's awesome and awesomely accurate!

In my experience, many of the aftermarket digital gauges are off, especially in the single digit pressures.

The exception is if you go with the high-end industrial digital gauges.

 

[entropy]

My Jeep is +2PSI higher on the dash than any gauge I've tried when around 15 PSI.

My recommendation. Get a cheap stick type gauge. Then confirm with your expensive gauge. If within +-3PSI have a beer and get on with your life. If the difference is higher then buy a new nice one and compare. Get rid of the one that's off.

You dont need to get that accurate, but if you are shooting for 12~15 psi. Make sure you are not grossly off so you don't go below 10 PSI.

 

[BDinTX]

I got an ARB deflator before a trip but didn't use it until we got to Colorado. I found it to be off by quite a bit too. I wrote a pretty critical email to them saying basically that they are a premium brand, their products should reflect that. They replied with an apology and said they'd send me a new one.

I got back to Texas and found that the ARB deflator was spot on. So was the new one they sent me... I think the altitude was throwing it off.

 

[roaniecowpony]

I wrung my hands over this topic of "an accurate tire pressure gauge" until I about pulled the skin off.

I read all I could find on available tire pressure gauge accuracy claims and was still frustrated.
So, I bought a lab quality industrial gauge that has an advertised accuracy that was acceptable (grade 1A +/- 1% of scale). Then I made my own tire gauge. You can buy the fittings on Amzon.

FWIW, I've found TPMS on my GMC and Jeep to be very accurate.

Ashcroft Process Pressure Gauges - Overview (ashcroft-gauges.com)

 

[No IFS]

Great thread I always wondered about this

 

[jbcrane]

I read all I could find ... then I made my own tire gauge.

You are my kinda guy. In the end - was it worth it? Do you get better (more accurate) readings with the Ashcroft?

 

[jbcrane]

I got an ARB deflator before a trip but didn't use it until we got to Colorado. I found it to be off by quite a bit too. I wrote a pretty critical email to them saying basically that they are a premium brand, their products should reflect that. They replied with an apology and said they'd send me a new one.

I got back to Texas and found that the ARB deflator was spot on. So was the new one they sent me... I think the altitude was throwing it off.

I'm in Colorado... I wonder if that's swinging my boojy ARB too? If so I can live with working out a correlation...

 

[Deleted member 59498]

If it is is +-1 or 2 pounds good enough.

 

[roaniecowpony]

You are my kinda guy. In the end - was it worth it? Do you get better (more accurate) readings with the Ashcroft?

"Was it worth it?" Of course not. LOL

"Do you get better (more accurate) readings with Ashcroft?" They're easier to believe.

I guess what I learned from all of this was that: if the gauge is reasonably close to the actual pressure, but repeatable, it's a good gauge. This is because we're all approximating the tire pressure we need. If I start running crazy low pressures, I may make a 0-15 psi gauge.

 

[Pig-Pen]

Id trust a mechanical gauge over digital. I about gave up on ny digital ones.

 

[lashlee]

I use Longacre analog gauges from my motorcycle racing days of the past. The instrument maintenance department at work has calibrated my gauges to around 0.5 psi at 34 psi. My ARB deflator was off by a fair amount until they tweaked it. Get something you like and stick with it!

 

[phageghost]

I got an ARB deflator before a trip but didn't use it until we got to Colorado. I found it to be off by quite a bit too. I wrote a pretty critical email to them saying basically that they are a premium brand, their products should reflect that. They replied with an apology and said they'd send me a new one.

I got back to Texas and found that the ARB deflator was spot on. So was the new one they sent me... I think the altitude was throwing it off.

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.

 

[jbcrane]

Well, here you go:

And it's @DiscountTire for the win.
To be fair - the Discount Tire stick was inconsistent - sometimes shooting up to over 40psi, sometimes reading 37. But... it did't cost $67 and it agrees with the Boss.
I can live with knowing ARB is 3# light and work it from there.

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