[Wolfus Aurelius]

This weekend we had a couple of cool mornings. On checking the TPMS before driving yesterday, I see it reads 30 psi in each tire. So I plug in the air pump and air each tire up so that the analog gauge appears to read about 38, and check each with my pencil gauge, which gives about 36-38. Imagine my surprise when I re-enter the car and TPMS tells me each tire is now at a big 32!

Which data source do you believe? I'm impressed with BMW's trip computer, for example. The fuel consumption since I reset it at my last fill-up said 26.6, and darned if that wasn't *exactly* the figure I got when I calculated the fill-up Saturday. So I'm inclined to think the TPMS might be pretty accurate. But my other two sources report differently. If I follow the TPMS, I guess I'd have to air the tires up to a pump gauge reading of about 40 in order to have TPMS report 34, which is where I'd like the tires to be. (The door placard says 32 all around.)

So which source do you rely on?

[MmmmmM2]

I gotta go with what my car is telling me. You just need to trust it? :

[chiefneil]

Pencil gauges and the built-in gauges on air pumps can be very inaccurate. Get a high-quality gauge that’s been certified/calibrated (ANSI B40.1) to use as a reference.

I use this one, but pick your favorite:

https://www.amazon.com/JACO-ElitePro...%2C192&sr=8-12

[Billfitz]

My gauge is on my inflator:

https://www.amazon.com/EPAuto-Heavy-...%2C1141&sr=8-5

It always agrees with my TPMS, and since it's part of the inflator there's no need to separately inflate and measure.

[Wolfus Aurelius]

Originally Posted by Billfitz

My gauge is on my inflator:

https://www.amazon.com/EPAuto-Heavy-...%2C1141&sr=8-5

It always agrees with my TPMS, and since it's part of the inflator there's no need to separately inflate and measure.

My inflator pump has an analog gauge too, which is why I'm asking about this. Like the man with two or more watches not knowing what time it *really* is, I have three inputs and am not sure which one to believe.

Sure, I can always pump the tires up a little higher, say to 40 on the pump gauge, check the TPMS, and let some air out if need be. I was wondering which source all of you go by.

[Billfitz]

If my inflator and TMPS didn't agree then I'd double check it by the gauges on my compressor.

[BeerHerder]

Doesn't the TPMS need a few minutes of driving to accurately measure, or is that the old style where it measured the difference in rolling diameter?

[Wolfus Aurelius]

Originally Posted by BeerHerder

Doesn't the TPMS need a few minutes of driving to accurately measure, or is that the old style where it measured the difference in rolling diameter?

Right; I started up the car, but the display reminded me of that. So I drove a block or so, and then it reported 30 all around.

[sspade]

My experience is very similar to OP...

Handheld gauges usually read a few PSI higher than what the car thinks the pressure is.

I just keep that in mind while filling and checking tires.

Reading through the lines... I guess that means we are using shitty, uncalibrated tire gauges?

[COKen]

As others have said, if you want to really know the pressure in your tires get yourself a good accurate race gauge. The TPMS is designed to recognize a change in pressure and don't really need to be able to tell you the correct pressure, just a change in the pressure from when you last set it. Many off the shelf gauges are just toys, and won't be accurate. A good race gauge should be calibrated to be accurate.

[Fliplegend]

My analog gauge is usually only 1 or 2 psi higher than what the TPMS says.

[BEM-S4]

Originally Posted by chiefneil

Pencil gauges and the built-in gauges on air pumps can be very inaccurate. Get a high-quality gauge that’s been certified/calibrated (ANSI B40.1) to use as a reference.

I use this one, but pick your favorite:

https://www.amazon.com/JACO-ElitePro...92&sr=8-12

I use that one as well. Added benefit if you inflate to max pressure for winter storage, the bleeder valve makes getting back down to operating levels very easy.

[kingmonkey8]

When I pumped up the tyres last time at the petrol station. I found the tpms to be fairly accurate.

Do you need to reset the tpms?

[Gen13 F36]

Tire pressure depend on what driver prefer. The recommended tire pressure on the car door card is manufacturer recommended for efficiency...this is how they get that mpg.

You can adjust to your preference.

[Billfitz]

Originally Posted by kingmonkey8

Do you need to reset the tpms?

Only if you want to change the reference pressure, or if you rotate the tires.

[kerny7678]

Originally Posted by Billfitz

Only if you want to change the reference pressure, or if you rotate the tires.

What does reference pressure mean?

[eluded]

I have a nice Air pressure gauge accurate to +\- 0.1 psi which is about as accurate as da Costco tire nitrogen air which is always within +\- .5 PSi of my TPMS. My TPMS sensor always accurate plus or minus half a psi

[Billfitz]

Originally Posted by kerny7678

What does reference pressure mean?

The pressure that the TPMS uses as the baseline. It will give a warning when pressure drops roughly 4 PSI below that. If you have your reference pressure set at 35 PSI you'll get a warning when it drops below 31 PSI or thereabouts. You wouldn't want that if you normally run at 32 PSI, it would give a warning every colder than normal morning. To prevent premature warnings and insure timely warnings fill the tires to your normal cold tire PSI, then do a reset to establish that as your reference pressure.

[Wires]

Originally Posted by kingmonkey8

When I pumped up the tyres last time at the petrol station. I found the tpms to be fairly accurate.

Do you need to reset the tpms?

If you change the tire pressure, I'd recommend resetting the tire monitor at the same time.

[Mr Podman]

Every car manufacturer will tell you their tire pressure numbers in their TPMS are approximate. Read your manual.

About 30 years ago I bought a real nice tire pressure gauge triple checked to be accurate. It's the bible.

Every little idiot in the stealership with a pencil gauge is ordered away from the vehicle.

Something you may find interesting about Honda's TPMS:

I live at 9,000 feet altitude. My Honda Pilot's TPMS system get's tricked and can't compensate for the altitude.

As a result, I have to drive around constantly with about 6 p.s.i. too much air in my tires so the TPMS system will think and read-out the "correct" tire pressure.

It was that way in my 2007 Pilot and it's that way in my 2018 Pilot. (Japanese are a model of consistency).

The BMW F32 TPMS system that lives right beside the Honda at 9,000 feet elevation, doesn't have that problem. But it also is a few psi inacurate compared to my gauge, as expected.




podman

[Gbtr6]

Also part of the equation is checking cold tire pressure, then driving. Pressures go up quite a few lb's when warm. And if you're having a track day, you need to check it when hot.

Perry

[skw]

And all this time I've been believing that my little pencil gauge was what's to go by. Off to Amazon to buy one of those aforementioned race/calibrated gauges...