What do you believe: TMPS or Tire Pressure Gauge?

[Zamorskii]

Yesterday was the first time I adjusted my tires pressure after I switched from my winter 19" to my summer 20" wheels set. I used a brand new digital tire pressure gauge and set fronts at 36 psi and rears to 44 psi.

This morning after driving for probably 20 minutes I checked the tire pressure in the Vehicle Status menu and the fronts read 31.5 and rears 41.9. Interestingly enough, both sides matched.

I earlier thought that perhaps my old gauge wasn't good any longer, hence I bought a new digital one, but I still see the discrepancy. Did anybody notice a similar issue, or am I alone?

[r33_RGSport]

Usually TPMS will have better measurement since they are calibrated from the factory, and their manufacturing process is certified internationally.

Tire pressure gauge will depends on how good of a brand they are. Even if it is digital, some of those cheaply may have big margin of error since they don't comply with international standard.

I personally invest in high end analog gauge that I use when I go to track. Its price usually around $100.

[double_j]

My TPMS is off by a little bit - two of them read low by about 2-3 lbs. The others are accurate.

I know my handheld gauge is more accurate because I use it across multiple cars and the BMW is the only one that's given me significant enough measurement differences to care about.

In addition, I've had the BMW TPMS tell me that my rears were inflated to different pressures when I had just confirmed with the handheld gauge that they were both identical. If it were a calibration issue with the gauge, TPMS should have told me that both rears were the same pressure, though it would have shown a different absolute pressure than reflected on my handheld.

The differences you get are odd though. Is there a chance that you somehow misread the digital gauge and set your tires to 26 and 34 instead of 36 and 44? Very strange that the TPMS would have shown that significant a drop, especially since it was a warm measurement.

[Zamorskii]

No, I was adding air to the tires last night and made sure I went up from ~ 30 to 36/44.
I can see at least that the TPMS reads consistently off from the tire pressure gauge, but not sure which is right. I should probably do a similar check on another car and see how far off it comes in.

I also noticed before when I did just winter/summer tire swaps that the TMPS reading was off after the tire shop changed the tires and supposedly set the correct pressures. Are their gauges junk too?

[Flying Ace]

there is a lot of variability in what you did.

1. You drove to the air pump on day 1, you then drove for 20 minutes this morning then checked the air. Warm air expands. A warm tire will read higher pressures than a cold tire. Both incidents were on warm tires with different ambient air temperatures, but on which measurement was it warmer?

You need to check pressures in the same environment. Check in the morning before you leave. That would be the best indicator...

2. if you pressures are slowly going down week by week, you actually may have a leak in the bead, likely due to improper sealing by the installer. I would stay on top of this.

[r33_RGSport]

I forgot to mention that 20 minutes of driving will change the pressure since the tire will be heated up.

Tire shop gauge usually uses whatever work gauge. Their margin is usually +/- 5psi.

When I pump my tires, I add air after the car parked in stable climate environment (in my case, my warehouse) for a few hours to cool down the tires. Or before I drive out from the garage from overnight parking.

Even when your car is outside, whichever side got hit by the sunlight, will have higher tire pressure. And if you add some air in this condition, once all 4 corners temperature are the same, the pressures will not be the same.

[iconoclast]

if you do not have a compressor at home or in your garage then i recommend you drive to pump and overfill evenly across all four tires. leave the pressure alone overnight. next morning on a cold tire adjust the pressures. if you would like to test the gauge afterward, drive to your destination and view in the iDrive then read the pressure manually with a gauge.

[Almaretto]

I use TPMS, electronic compressor, & analogue hand check. They all work.

Biggest issue with in car system is that only one tire can be significantly low to alert driver (ie all four equally 4-6 PSI lower than recommended and no warning).

[iconoclast]

same here. i noticed my analog gauge is off by an even 2psi from iDrive and compressor but it is seventeen years old so i have to guess that the analog gauge is the erroneous indicator.

[F15GorDe]

Switching from one set to another set of tires and wheels requires a "reset" of the TPMS system. I see no mention of this in the OP's post.

My TPMS readings match my gauge readings I take on COLD tires ONLY with my 40 year old gauge. Think this is why I have over 35k miles on the PIRELLIs that were on my F15 when I picked it up at in Spartanburg.

I set mine at 36F and 44R. Display is usually within a few tenths, occasionally they match.

[Zamorskii]

Originally Posted by F15GorDe

Switching from one set to another set of tires and wheels requires a "reset" of the TPMS system. I see no mention of this in the OP's post.

My TPMS readings match my gauge readings I take on COLD tires ONLY with my 40 year old gauge. Think this is why I have over 35k miles on the PIRELLIs that were on my F15 when I picked it up at in Spartanburg.

I set mine at 36F and 44R. Display is usually within a few tenths, occasionally they match.

Off course I did a reset. I measured the pressure in the morning and then checked against the TPMS system reading right after I started to drive. Both are within 2 psi. I think my flaw was that I added air to hot tires.

[Mexecutioner]

My digital gauge is off by 0.7psi compared to the TPMS, it's always very consistent. I pump air on a weekend morning after the car has been garaged overnight. I just pump 0.7 psi more to get the right measurement on iDrive.

[IndyJer]

Originally Posted by Almaretto

I use TPMS, electronic compressor, & analogue hand check. They all work.

Biggest issue with in car system is that only one tire can be significantly low to alert driver (ie all four equally 4-6 PSI lower than recommended and no warning).

TPMS isn't looking for tire pressure to drop below a certain recommended pressure. It is watching for any one of the tires to have a pressure drop of a certain % from the pressure at the time the system was last reset. The pressure drop its looking for is usually around 25%. So if your tire pressure was 40psi when you last reset the TPMS system, a warning will occur when that tire drops to 30psi.

This is why it's important to always reset TPMS every time you adjust tire pressures. The reset is telling the system what the new starting pressure is for each tire, if pressure drops 25% or more from that pressure you will get a warning.

[Flying Ace]

Originally Posted by Zamorskii

Off course I did a reset. I measured the pressure in the morning and then checked against the TPMS system reading right after I started to drive. Both are within 2 psi. I think my flaw was that I added air to hot tires.

Yep... Didn't you take chemistry lab classes or something in school? You always measure things in a consistent manner.

[Flying Ace]

Originally Posted by Mexecutioner

My digital gauge is off by 0.7psi compared to the TPMS, it's always very consistent. I pump air on a weekend morning after the car has been garaged overnight. I just pump 0.7 psi more to get the right measurement on iDrive.

On a F82, 0.8 psi off will make you crash your car when you leave cars and coffee

See my avatar for proof. Lorde
1MOREMOD
EJT86

[Almaretto]

Originally Posted by IndyJer

TPMS isn't looking for tire pressure to drop below a certain recommended pressure. It is watching for any one of the tires to have a pressure drop of a certain % from the pressure at the time the system was last reset. The pressure drop its looking for is usually around 25%. So if your tire pressure was 40psi when you last reset the TPMS system, a warning will occur when that tire drops to 30psi.

This is why it's important to always reset TPMS every time you adjust tire pressures. The reset is telling the system what the new starting pressure is for each tire, if pressure drops 25% or more from that pressure you will get a warning.

I have seen a vehicle with recommended 41/36 drop all tires to 28-30 PSI and without warning on dash.

The IDrive screen displays actual readings of temperature and pressure.

I maintain suggested pressure of my tires and do not reset each time I adjust.

[IndyJer]

Originally Posted by Almaretto

I have seen a vehicle with recommended 41/36 drop all tires to 28-30 PSI and without warning on dash.

The IDrive screen displays actual readings of temperature and pressure.

I maintain suggested pressure of my tires and do not reset each time I adjust.

Well say the tires were at 36/32 when TPMS was reset. Tires are then inflated to 41/36. TPMS still thinks they are at 36/32 so won't throw a warning until 27 or 24 is reached. It's all about telling the system what the starting pressure is, that's what the reset is for. If you are always keeping tires at the same pressure the TPMS was last reset, then you don't necessarily need to reset and should see warning when a tire drops 25%.

http://www.tpmsmadesimple.com/how_tpms_works.php

[Almaretto]

Originally Posted by IndyJer

If you are always keeping tires at the same pressure the TPMS was last reset, then you don't necessarily need to reset and should see warning when a tire drops 25%.

That is all I am saying. I always keep my tires at designated pressures, so I do not see the point is resetting.

[PatrickC23]

So what are people using for optimal tire pressure for the 21s?

I just swapped my summers back and forgot what I ran them at previously, the wheel shop set them at 36 but I don't feel as if the ride is how I remembered it. I read in another thread 38/38 is optimal. Load is only two front passengers normally.

[Almaretto]

Originally Posted by PatrickC23

So what are people using for optimal tire pressure for the 21s?

I just swapped my summers back and forgot what I ran them at previously, the wheel shop set them at 36 but I don't feel as if the ride is how I remembered it. I read in another thread 38/38 is optimal. Load is only two front passengers normally.

I do not keep my 19" at even pressure. The door jam gives recommended pressures, manual has other suggestions, and tire specs should give suggests.

[Flying Ace]

Originally Posted by PatrickC23

So what are people using for optimal tire pressure for the 21s?

I just swapped my summers back and forgot what I ran them at previously, the wheel shop set them at 36 but I don't feel as if the ride is how I remembered it. I read in another thread 38/38 is optimal. Load is only two front passengers normally.

just double check your manual.

The X5 has multiple settings dependent on wheels size, load weight and speed you will travel. On my E70 OEM 20s, I prefer the setting in between the minimum and maximum.

[GeeRam]

At 3 months old and 4k miles, my pressure warning tripped last night, and checking the pressure in IDrive, it listed the fronts at 2.1 bar and the rears at 2.3 bar (sticker on door pillar states 2.2 for front and 2.4 for rears) so activated with a mere .1 bar drop.....?

My suspicion was raised, so out comes the expensive tyre pressure gauge from my racing/rallying days, and fronts say 2.5 and rears say 2.7......

Ok, try another gauge, a less expensive one, but still not a cheapo, and almost the same readings.
Hmmmmmmnnnnnnn..........

Just for shits and giggles I put in another 0.3/4 bar in each tyre from the foot pump so fronts are now reading 2.8 and 3.0 on the gauge, and did a reset (how long does that take - WTF!!) and eventually, the IDrive readings show........2.2 front and 2.3 rear..........

So, it looks like the TPMS is about as reliable as a chocolate teapot......

So much for technology.