TPMS Accuracy

[user 8033]

I'm curious to hear the experience of others with the TPMS system. Here's what always happens with my 2020 REX.

Tonight I looked at the tire pressure status and it said they were 35 front and 39 rear. I have the 20" rims. According to the door jam sticker, they're supposed to be 39 front and 44 rear. So I got my trusty tire gauge out and the bike pump and topped them off to 39 front and 44 rear.

The gauge has always been historically accurately on two other recent vehicles.

I hit the reset, as usual, it takes forever, Wasn't bad getting up to 99%, but it took several miles of driving to get from 99% to 100%. When it finally cleared, it reported the front was 37 psi and the rear was 41 psi. Not true according to the tire gauge.

WTF? Anyone else having similar behavior?

 

[Arm]

Yes...I constantly am about 1 to 2 PSI off.

 

[youngwilliam]

Same here on a 2018 BEV with 19" rims, 1-2 lbs less per TPMS. I typically inflate tires when cold to 34 front/42 rear. After a reset and driving a few miles, they register ~32 and 40, respectively. You'd think it would be higher as the tires heat up while driving, at least it's consistent. I appreciate the onboard tire pressure monitoring, but I'd rather rely on manually checking tire pressures.

 

[user 8033]

Interesting how so many people seem to have the exact same issue with the TPMS.

I like the feature when it works correctly. My 2021 Jeep Wrangler was right on the money. Simple toggle down on the steering column and I had correct numbers. My 2019 Corvette was really accurate. And my VW Golf R was also really good.

BMW should be looking into this.

 

[user 8033]

Drove my 2021 BEV i3 for the first time tonight since bringing it home from the dealer. Exact same problem with the TPMS as my 2020 i3 had. It's been sitting since late July, so the tires had lost a few pounds. When we took off, I checked the readout and it said low 30's in the front and high 30s in the rear. So I let the car cool down for a couple of hours in the shade, then inflated the front to 39, and back ones to 44. Did the reset for several miles, when it concluded, it said 36 psi in the front and 41 in the rear. This i3 TPMS really is whacked.

This 2021 has the 19" wheels. What a difference in the ride. Hitting bumps with the 20" wheels felt like the car was being assaulted. Huge difference in overall ride quality. Very pleased with the decision to forego the bigger rims and tires this time. Although, I do think the 20" rims look nicer.

 

[user 8033]

Had my "how are you loving your new 2021 i3?" phone call from BMW's product team this week. Decided to nail the caller with the TPMS issue, explaining that I have the same problem on both a 2020 and 2021. Also told her other people confirmed having the issue. BMW's response was take it to my local dealer. Anyone tried addressing it yet with any of their service departments? I'm not in the mood for the customer experience. Had a door handle replaced on the 2020. Had the car given back with 32 PSI in all four tires :x

 

[alohart]

I view the TPMS system as a warning system for inflation pressure changes, not for accurate pressure readings. Inflation pressure varies considerably with temperature, so the inflation pressure can change considerably while driving. Every couple of months, I check the inflation pressure with an accurate pressure gauge after our i3 has been parked overnight. I either release or add air to attain my inflation pressure goal. I depend on the TPMS system to warn me when the inflation pressure of a tire changes relative to other tires which usually indicates a leak.

 

[user 8033]

The TPMS system routinely changes readings, I check it whenever we drive the car. For example, when it's 110 outside, it reads that the tires are pretty much correct in pressure, although that's not an accurate reading and they are higher due to the extreme temps. On the other hand, now that it's sometimes hitting 70 something at night, it'll read the tires are 35 or 36 psi when in reality they are around 39. It's just a flat-out badly designed system that really doesn't work, short of your scenario of having rapid pressure loss. That's yet to be tested in my case, to see if that even works? As I've pointed out, this isn't the case with other manufacturers. Each 10-degree change in air temp is supposed to equate to plus or minus 1 psi unless you have nitrogen in them.

 

[alohart]

Each 10-degree change in air temp is supposed to equate to plus or minus 1 psi unless you have nitrogen in them.

For what it's worth, the pressure of all gases changes the same with temperature changes. Nitrogen behaves no differently from air which is 78% nitrogen. Exceptions would be when water vapor is present in the inflation gas and the inflation gas temperature drops below the freezing point of water or rises above the boiling point of water. Nitrogen is more likely to be dried than air when used to inflate tires. However, that wouldn't matter under most conditions.

All gases follow Gay-Lussac's law which states that a pressure change is proportional to the change in absolute temperature (the temperature scale with absolute zero as its minimum). So P2 = P1 * T2 / T1. If P1 is 35 psi and T1 is 20 ºC (293 ºK) and the temperature increases 10 ºC, the pressure would be 35 psi * 303 ºK / 293 ºK = 36 psi. As you stated, a 10 ºC temperature change would result in a 1 psi pressure change. A 10 ºF temperature change would result in about ½ psi pressure change.

 

[Neil]

I've found the TPMS readings to be accurate to about 1 or 2 psi which is good enough for me. Of course, the pressures will vary with temperature so the readings will change. I also enabled the temperature readings via coding as a curiosity and although I'm not sure of the absolute accuracy of the readings, they seem believable to me and I can see the temperature rise in the tires on one side facing the sun for example.

 

[Techturbo]

Same observation on my 2014 showing 2 - 3 PSI lower even after a reset. I have a 2017 MINI cooper (made by BMW) with the same observation... hard to explain why it is like that.

 

[Arm]

I understand that the TPMS can vary based on temperature, etc. But I do wish we could calibrate them to the pressure we pump them to.

I've a 2019 i3s BEV and I'm always low 2 psi. I set mine right in the money per factory spec on a cool day, undriven. Thus, it would be nice if I could set my pressures manually after pumping my tires to the the correct pressures.