TPMS not at all accurate

[Calson]

This morning my dash showed all four tires at 36 psi and so OK. When a Ford tech arrived to take care of a recall fix he also used his digital gauge to check the pressure in the tires. His gauge showed all four as having only 33 PSI. As all four tires were off either his gauge was not accurate or the TPMS is off.

Tough to know which gauge one can trust as none are calibrated with a standard. I do know I will be checking my tires with my hand gauges more often and not trusting the dash display to monitor the tires. There are a great many inaccurate pressure gauges on the market.

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[powerboatr]

i have found if truck sits...say over a few hrs
the dash , nor ford pass shows accurate psi until you drive a few miles

i read something explaining it in the manual .
the sensors dont transmit until movement is detected so the psi it shows is probably what it was when you last drove . in a simplified answer

at least you didnt get this in ford pass

 

[Gros Ventre]

I have found mine to be within a pound of a handheld gauge.

 

[nvabill]

i have found if truck sits...say over a few hrs
the dash , nor ford pass shows accurate psi until you drive a few miles

i read something explaining it in the manual .
the sensors dont transmit until movement is detected so the psi it shows is probably what it was when you last drove . in a simplified answer

at least you didnt get this in ford pass

Yes, I think you are correct, if you come home with 36 showing in the tires when it sits overnight it can go down to 33 due to cold but TPMS will still show 36 unless you drive it.

 

[Calson]

I add air almost monthly and use a Viair 12V air compressor that adds very hot air to the tires. I fill until the tires are at 38 PSI and then let them sit overnight and let out air to get down to 35 PSI using a hand held gauge.

When I am adding air the dash display will jump up by 1 or 2 PSI during the process and this is with the truck parked in my driveway.

I expect the tires that are facing the sun to have a higher PSI than the tires that are shaded by the vehicle. This morning the readings were taken before the sun lit the tires and so indicated the difference in the air inside the tires and the ambient air temperature which was the same.

I have never relied on the TPMS readings in the past and will certainly not do so in the future.

 

[amschind]

PV=nRT ; P=pressure V=volume n=number of moles of gas (billiard balls, so 1 molecule of N2=1 for this purpose) R=gas constant and T=temperature

Thus, if your target is defined in P, V and R are fixed, T varies (based upon driving, sun, ambient temperature, et c) and you have control over n (your air compressor forces more molecules of N2 and O2 inside the tire), then you have to set boundary conditions for T so that you adjust n correctly. Those boundary conditions don't have to be one specific condition, but they do have to be consistent between design spec, use, filling and measurement.

 

[Trailrated]

If I may piggyback on this thread..... Do 2023 XL's have TPMS viewable on the dash? I can't find that setting so I'm guessing I'll just get a basic dummy light and I gotta guess which tire is low

 

[Aron]

I agree with others than mention that the TPMS will display the last sensor reading from the previous day when you first turn on your vehicle, and that you'll have to drive a few miles to get a new, accurate reading for the current tire pressure and temperature. I'm guessing that it does this to limit constant sensor reading while the vehicle is stationary, to preserve battery life of the sensors.

However, even if your readings are ~10% off (33 vs 36 psi), I wouldn't consider that "inaccurate". I doubt that the sensor precision is intended to be spot on to the correct pressure. Rather, the TPMS is a safety system designed to alert you if you're WELL below (or above) recommended pressures, since that's when tire inflation issues can cause crashes. The difference between 33 and 36 psi won't affect much from a safety standpoint. I wouldn't rely on the TPMS to dial in the pressure when adding air, but they seem pretty accurate for the purpose of a safety warning system.

Maybe I'm in the minority, but I don't make a habit of ensuring that my tire pressures are always precisely 36psi. Every once in a while (like every few months), I'll check the TPMS screen and verify that they're not crazy low, but that's about it. If I saw that my tires were reading 33, I wouldn't think twice about it, especially if all 4 were in the same ballpark. On the other hand, if one were reading 33 and the other 3 were 38 (after heating up), then I'll keep an eye on it to make sure that it's not getting worse, and add air to the low one at my next convenience.

@Calson , I admire your dedication to precision tire inflation, but I don't go to those extremes.

 

[Calson]

I take it you have not added air or simply seen the difference in tire pressure indicated between the tires in the sun and those in the shade? If you had then you would know that the pressure is measured when the truck is on and it does not need to be moving for the readings to be sent and received.

When I add air with a compressor I can watch the readings change in real time. I do not need to drive the truck to get new readings.

Bottom line is I will be relying on my hand gauges and not the TPMS. The only reason this system was mandated by the federal government was to improve fuel economy painlessly by having drivers drive fewer miles with their tires with too little tire pressure which increases the fuel being burned.

 

[Aron]

I take it you have not added air or simply seen the difference in tire pressure indicated between the tires in the sun and those in the shade? If you had then you would know that the pressure is measured when the truck is on and it does not need to be moving for the readings to be sent and received.

When I add air with a compressor I can watch the readings change in real time. I do not need to drive the truck to get new readings.

Bottom line is I will be relying on my hand gauges and not the TPMS. The only reason this system was mandated by the federal government was to improve fuel economy painlessly by having drivers drive fewer miles with their tires with too little tire pressure which increases the fuel being burned.

I have not added air while watching the TPMS, so I didn’t realize that. (Like you, I rely on my analog gages when adding air.) It works that way with my trailer TPMS, and with some other cars that I’ve had, so I guess I subconsciously assumed that it worked the same way.

Regardless, the TPMS is mandated by NHTSA Federal Motor Vehicle Safety Standard 138. TPMS systems were mandated after there were a significant number of rollover crashes by SUVs (back in the 90s maybe—I don’t recall the timing) that were attributed to low tire pressure.

TPMS might also have fuel efficiency benefits, but that’s not NHTSA’s mission, and it’s not an EPA regulation (who write the rules about fuel efficiency).

 

[Grafx36510]

I have not added air while watching the TPMS, so I didn’t realize that. (Like you, I rely on my analog gages when adding air.) It works that way with my trailer TPMS, and with some other cars that I’ve had, so I guess I subconsciously assumed that it worked the same way.

Regardless, the TPMS is mandated by NHTSA Federal Motor Vehicle Safety Standard 138. TPMS systems were mandated after there were a significant number of rollover crashes by SUVs (back in the 90s maybe—I don’t recall the timing) that were attributed to low tire pressure.

TPMS might also have fuel efficiency benefits, but that’s not NHTSA’s mission, and it’s not an EPA regulation (who write the rules about fuel efficiency).

It was the Ford Explorer/Firestone Tire if I recall correctly.

 

[JefCo23]

Air gauges rarely all read the same. It’s only a few psi difference.

 

[bfastr]

the manual pretty clearly says its not to be used as accurate data.
like others, I use it as a reference. mainly looking for all of them to be in the same range.

 

[Boyk1182]

I have checked with several gauges, one being an expensive digital gauge that gets to a tenth of a pound. My truck reads 2-3 PSI higher than the gauges.

If you pull down to refresh the FordPass app, it will give you the current reading, otherwise it’s probably from a while ago.

 

[Buyer2021]

the manual pretty clearly says its not to be used as accurate data.
like others, I use it as a reference. mainly looking for all of them to be in the same range.

IMO the true utility and original intent when TPMS was mandated by the feds is to warn a driver of puncture-related pressure loss while underway. For that one doesn't need absolute-value accuracy, just relative pressure change indication, and the system provides that quite adequately.

I do appreciate that the Ford Accessory integrated trailer-TPMS shows temperature as well as pressure, and I wish the vehicle TPMS did the same. Again, I'm not expecting absolute-value temperature accuracy, just an indication of relative temperature increase, which can be a valid early-indicator of underinflation or other trailer-axle issues (e.g. misalignment causing excessive tire scrubbing).

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