TPMS pounds reading versus tire gauges.

[KDeline]

My readings on 5 different BMW's using the OEM TPMS seem to be about 6 pounds low as compared to various tire gauges, cheap or expensive. I run at 50 pounds per gauge to get a reading of 42 to 44 on the TPMS. Curious what others get.

[9Mary7]

I use a calibrated gauge and ignore to TPMS....until it warns of a puncture.

[wbw6cos]

Hi Ken,  currently I have Metzelers on my RT and they read what my air gauge indicates, which is surprising; 36 front and 42 rear. 

 

The previous set of Michelin Road 6 GT's and the Road 5 GT's before that were always off by about 3 or 4 lbs; the TPMs were lower than the tire gauge, which is a Tiretek brand, 0-60psi.

 

I always have checked them cold and rely on the TPMs for significant air loss while riding.

[9Mary7]

10 hours ago, wbw6cos said:

I always have checked them cold and rely on the TPMs for significant air loss while riding.

[Lowndes]

Most of my old junk didn't come with TPMS but I've installed aftermarket systems on everything including the lawnmower.  Several close calls (bikes and cars) have convinced me of the value of TPMS.

 

If you watch the pressures on the display while riding you notice constant variation in the temps and therefore pressures.  It can range 5 or 6 PSI during the day, depending on ambient temps, sunshine, bike speed, even the direction you are riding.  Yep.  If the front tire has direct sunshine from either side it gets warmer and the pressure WILL go up.  Pressure is directly related to temperature.  After seeing all this constant changing I don't worry so much about the exact pressure on starting out in the morning.   If it's cooler I know it will be a little lower in pressure but will warm up.  If you put 42 PSI in a tire at 30° F in the early morning, it might get up to 50 or more PSI during the day.  It's happened to me.  And it set off the alarm.

 

All of my pressure guages (one on the chuck, a small "stick guage" on each bike, and a very good hand held 2.5" oil filled gauge never agree exactly with any of the TPMS sensors or each other, but as they say, "everything is relative".

 

Remember, Confucius say, "Man with two watches NEVER KNOW what time it is."

 

 

[lather]

The TPMs on my 13 RT read about 1 psi higher than my digital gauge. It is my understanding that these units  display a psi corrected for temperature. My 2017 Multistrada does not have OEM TPMs and I am using FOB Tire 2 on it. These display on a smart phone app and are not corrected for temperature. The app also displays the tire temperature. I find it interesting to watch the psi climb as the tire temperature climbs on starting a ride. On a cool road the psi will climb about 5 degrees in front and 7 degrees in rear after about 8 miles. On a warm sunny day psi will go even higher.

[dirtrider]

14 hours ago, KDeline said:

My readings on 5 different BMW's using the OEM TPMS seem to be about 6 pounds low as compared to various tire gauges, cheap or expensive. I run at 50 pounds per gauge to get a reading of 42 to 44 on the TPMS. Curious what others get.

Morning  KDeline

 

Is this an OEM BMW TPMS or an aftermarket? (can make a difference)

 

If you are working with a factory BMW TPMS then do your testing at exactly 68°f as that is the only temperature that the BMW TPMS will match a remote gauge. 

 

The BMW system is calibrated & CORRECTED to 68°f, so all it's readings are corrected to what the tire pressure would be at 68°f.

 

Or you can sort of figure it in your head at 1 psi change per 10°f in tire temperature change. So at 78°f (tire temperature) the TPMS should read around 1° lower than your gauge. 

 

What is your altitude? The TPMS reads with it's sensor inside the tire pressure cavity and most external gauges read (basis it's reading) against atmospheric pressure (obviously lower at higher altitude).

 

Some tire dealers have a master (calibration) gauge to check their other service gauges against. If you have one near you then see if they will allow you to check the accuracy of your gauges.

[Skywagon]

14 hours ago, wbw6cos said:

I always have checked them cold and rely on the TPMs for significant air loss while riding.

 

That ^  I keep my display on tire pressure and watch it occasionally.  The one time I did have a flat while riding, the display and warning lights caught my attention before it went completely flat.

[Joe Frickin' Friday]

5 hours ago, dirtrider said:

The BMW system is calibrated & CORRECTED to 68°f, so all it's readings are corrected to what the tire pressure would be at 68°f.

 

 

What is the rationale for doing this?  If I fill my tires to 40 psi at 68F, surely they're not expecting me to ride around with my tires at 37.3 PSI when it's only 32F out?

 

5 hours ago, dirtrider said:

What is your altitude? The TPMS reads with it's sensor inside the tire pressure cavity and most external gauges read (basis it's reading) against atmospheric pressure (obviously lower at higher altitude).

 

 

 

A long time ago I had an aftermarket TMPS on my 1100RT. I seem to recall that it had an ambient pressure sensor in the display unit so that it could take the absolute pressure from the tire sensors and properly display an accurate gauge pressure regardless of altitude.  Are there TPMS systems that just assume a constant ambient pressure (somewhere between the 14.7 psi at sea level and the 8.7 PSI on Pike's Peak) and subtract that from the sensor's absolute pressure reading?  

[dirtrider]

49 minutes ago, Joe Frickin' Friday said:

 

 

What is the rationale for doing this?  If I fill my tires to 40 psi at 68F, surely they're not expecting me to ride around with my tires at 37.3 PSI when it's only 32F out?

 

 

 

A long time ago I had an aftermarket TMPS on my 1100RT. I seem to recall that it had an ambient pressure sensor in the display unit so that it could take the absolute pressure from the tire sensors and properly display an accurate gauge pressure regardless of altitude.  Are there TPMS systems that just assume a constant ambient pressure (somewhere between the 14.7 psi at sea level and the 8.7 PSI on Pike's Peak) and subtract that from the sensor's absolute pressure reading?  

Afternoon Mitch

 

 

What is the rationale for doing this?  If I fill my tires to 40 psi at 68F, surely they're not expecting me to ride around with my tires at 37.3 PSI when it's only 32F out?--  This is not any different than some automobile OEM systems. A lot of current OEM auto/ light truck  systems are also temperature compensated. One of the things that drove the temp compensation addition (at least at the auto company that I work for was allowing the low tire pressure warning to be more accurate, it it didn't have the compensation then the driver could get a low pressure warning at low temperature drive-off) 

 

A long time ago I had an aftermarket TMPS on my 1100RT. I seem to recall that it had an ambient pressure sensor in the display unit so that it could take the absolute pressure from the tire sensors and properly display an accurate gauge pressure regardless of altitude.  -- This could have been true if the wheel sensors attached  outside of the pressure chamber as knowing the atmospheric pressure would make the system  more accurate.

 

With the pressure sensor inside of the pressure chamber (like most OEM systems)  there would probably be no need for the atmospheric  pressure compensation as it wouldn't need the atmospheric pressure offset to read true. 

 

Are there TPMS systems that just assume a constant ambient pressure (somewhere between the 14.7 psi at sea level and the 8.7 PSI on Pike's Peak) and subtract that from the sensor's absolute pressure reading?-- I would imagine so as that would be a very cheap (aftermarket) probably valve cap type unit that doesn't need to meet Federal  low tire pressure warning specifications as well as no atmospheric pressure compensation baked into the unit. 

 

Still a lot better than the (economy car) original low tire pressure warning systems that used the ABS wheel speed signals to electronically judge that a tire was low based on that tire's rotation per mile vs the other 3 wheels. It worked to meet the Federal required low tire warning law but not without some false alarms. 

[KDeline]

So then I guess part of my question that I did not include, would be which gauge would be correct? Do I run my tire pressure according to the reading of the TPMS, or assuming a calibrated air gauge is correct, run off of that? Since these are such high priced, high end quality machines using only the best components,  (😆 sometimes I crack myself up) you would think that you would use the reading they display. Personally, I’m not that anal give or take 5 pounds, and I seem to get OK tire mileage. Nothing like I used to get on the old airheads back 30 years ago. Just curious what everybody else was getting compared to their manual gauges.

[dirtrider]

11 minutes ago, KDeline said:

So then I guess part of my question that I did not include, would be which gauge would be correct? Do I run my tire pressure according to the reading of the TPMS, or assuming a calibrated air gauge is correct, run off of that? Since these are such high priced, high end quality machines using only the best components,  (😆 sometimes I crack myself up) you would think that you would use the reading they display. Personally, I’m not that anal give or take 5 pounds, and I seem to get OK tire mileage. Nothing like I used to get on the old airheads back 30 years ago. Just curious what everybody else was getting compared to their manual gauges.

Evening KDeline

 

That might be in your rider manual (it is spelled out in a lot of automobile owners manuals), most say to NOT use the onboard TPMS to set tire pressures as there can be reading delays & as well as some specific accuracy issues.  It is difficult to set your tire pressures with the wheel spinning & most OEM TPMS sensors don't wake up until the vehicle has moved a certain distance, so sometimes the pressure shown on the dash can be last remembered data.  

 

Personally I wish the OEM systems allowed toggling between temperature compensated & direct reading but that would mess up a lot of riders that are not educated on tire pressure vs tire temperature & would probably require a lot more elaborate (re expensive) TPMS system to allow it it to meet Federal low tire pressure warning standards (to be accurate & meet the fed standard this is typically based on compensated).    

[Joe Frickin' Friday]

31 minutes ago, dirtrider said:

One of the things that drove the temp compensation addition (at least at the auto company that I work for was allowing the low tire pressure warning to be more accurate, it it didn't have the compensation then the driver could get a low pressure warning at low temperature drive-off) 

 

If my car's door jamb says the tires are supposed to have 35 psi in them (measured cold, i.e. with tires at current ambient temp), and the TPMS is telling me "35 psi" but the tires are actually 10% low because it's 15F out instead of 68F, aren't my tires underinflated?  Wouldn't I want the low-pressure warning to be triggered under that circumstance?

 

35 minutes ago, dirtrider said:

A long time ago I had an aftermarket TMPS on my 1100RT. I seem to recall that it had an ambient pressure sensor in the display unit so that it could take the absolute pressure from the tire sensors and properly display an accurate gauge pressure regardless of altitude.  Are there TPMS systems that just assume a constant ambient pressure (somewhere between the 14.7 psi at sea level and the 8.7 PSI on Pike's Peak) and subtract that from the sensor's absolute pressure reading?-- This could have been true if the wheel sensors attached  outside of the pressure chamber as knowing the atmospheric pressure would make the system  more accurate.

 

With the pressure sensor inside of the pressure chamber (like most OEM systems)  there would probably be no need for the atmospheric  pressure compensation as it wouldn't need the atmospheric pressure offset to read true. 

 

Not sure what you mean by reading "true".  There's absolute pressure, and there's gauge pressure (= absolute pressure in the tire minus ambient pressure outside the tire), and the latter is what I would expect a TPMS to report in order to be consistent with handheld tire gauges, door jamb stickers, and sidewall pressure ratings.  The system I had used sensors that were completely contained inside the tire, so they would only have been able to measure absolute pressure inside the tire; the system could not have correctly displayed gauge pressure without a separate external sensor to measure local atmospheric pressure.  

 

[dirtrider]

42 minutes ago, Joe Frickin' Friday said:

 

If my car's door jamb says the tires are supposed to have 35 psi in them (measured cold, i.e. with tires at current ambient temp), and the TPMS is telling me "35 psi" but the tires are actually 10% low because it's 15F out instead of 68F, aren't my tires underinflated?  Wouldn't I want the low-pressure warning to be triggered under that circumstance?

 

 

Not sure what you mean by reading "true".  There's absolute pressure, and there's gauge pressure (= absolute pressure in the tire minus ambient pressure outside the tire), and the latter is what I would expect a TPMS to report in order to be consistent with handheld tire gauges, door jamb stickers, and sidewall pressure ratings.  The system I had used sensors that were completely contained inside the tire, so they would only have been able to measure absolute pressure inside the tire; the system could not have correctly displayed gauge pressure without a separate external sensor to measure local atmospheric pressure.  

 

Evening Mitch 

 

Quote

If my car's door jamb says the tires are supposed to have 35 psi in them (measured cold, i.e. with tires at current ambient temp), and the TPMS is telling me "35 psi" but the tires are actually 10% low because it's 15F out instead of 68F, aren't my tires underinflated?  Wouldn't I want the low-pressure warning to be triggered under that circumstance?

No necessarily, THIS is a very gray area & something the auto industry wheel & tire engineers have vacillated back & forth on for years. It kind of falls under the same heading of using the same (specified) tire pressures on both the front & rear tires on some vehicles. Is that correct? Definitely not but it is understandable for most (regular) drivers.  So as long as using the same ft/rr is safe on that vehicle then that is done on a lot of automobiles.

 

For the most part  it is accurate enough to meet standards. 

 

Most tell you "not" to use the TPMS for pressure setting & also figure that most will set the cold pressures in the 45°-75° range. Sure you can have the outlier but setting tire pressures at 0°f or even 15°f gets iffy to begin with as there is a chance that a tire valve core can stick in due to freezing moisture issues.

 

If the OEM door sticker says 35 psi cold that is definitely figured with a  safety fudge factor as the OEM's don't expect a driver to check & fill their tires every day before they drive off. Or to do it at exactly 68°f. 

 

When we do specific handling or safety testing where I work we even drop the vehicle off the hoist onto the ground so vehicle weight is on the tires.  Does that make a lot of difference? No, but we need to be precise. We also check tire pressure gauges against a calibrated master before & sometimes after any (important) test pressures are set.  

 

 

The other factor is  tire gauges, most  (non engineering or scientific) people do not maintain accurate tire pressure gauges. 

 

Quote

Not sure what you mean by reading "true".  There's absolute pressure, and there's gauge pressure (= absolute pressure in the tire minus ambient pressure outside the tire), and the latter is what I would expect a TPMS to report in order to be consistent with handheld tire gauges, door jamb stickers, and sidewall pressure ratings.  The system I had used sensors that were completely contained inside the tire, so they would only have been able to measure absolute pressure inside the tire; the system could not have correctly displayed gauge pressure without a separate external sensor to measure local atmospheric pressure. 

External gauges remove atmospheric pressure from the reading. As I'm sure you know the 0 on a handheld gauge is actually not 0 but reflects an inherent atmospheric pressure offset (not sure what the number actually is). The internal TPMS sensors simply read in absolute pressure then my guess is that is modified using the atmospheric sensor output from the fueling computer to reflect the atmospheric pressure off-set . 

 

 

[Skywagon]

My wifes Lexus has TPMS.  Tires are supposed to be at 32psi cold.  I set them with external digital tire gauge.  When we have the occasional cold snap here, she will get in the TPMS will be alarming.  I check it again with gauge and it will usually be around 29psi when that happens.

 

On the bike I only set once a day and sometimes once every 3-4 days depending on riding.  I set with a digital external gauge and forget about it.

[Hosstage]

Timely topic. I have a moderate trip to make today and tomorrow. I don't check the tires often enough in my wife's car, she has tpms and only makes short runs every few days. Because we will be traveling, and it's about 10°F, I will be setting them at factory 35lbs. before we head out. I expect them to be at about 30-32 when I check them, even though I set them to 35 last month when it was 40°. I will check tpms to see what it says once underway.