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Let's talk tire pressure :)

Rock&Roll

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I can't imagine ever thinking about this stuff if I didn't have the cool gauge that shows the tire pressure on each tire at any given moment.
Crazy right :headbonk:


Anywho, my tires read 30 -31 in the morning at 30-40 degrees.
If I get in my car in the afternoon when it's warmer at 60 degrees the tires might read 31-32

Drive for 20 -25 minutes and the pressure is up to 34-35ish



Any Thoughts ?



Wondering if I should lower tire pressure a little more or :shrug:

Stupid Tire Gauge :headbonk: Never thought about this stuff with my RAV4 :crazy:
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TexasRebel

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The TPMS doesn't have the most accurate of sensors, and temperature and barometric pressure certainly play a role.

I've found that when my dash says 32psi my gauges all say 28-30. When my gauges say 32 psi the dash will say 34-36, and on warmer days or long drives the pressures will increase to 40 on the dash.

The TPMS is a good quick & dirty way to see a problem, but it'd be better if there was just a green/yellow/red type indicator.
 

airfuel

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From tire buying guide:
"A good estimate to use when comparing tire pressure to air temperature is for every 10 degrees F, tire pressure will adjust by 1 psi. For example, if the outside air temperature increases 10 degrees, the tire pressure will increase by 1 psi."

This just takes into account outside air temperature. Driving causes flex and heat and also raises pressure.

Just set pressure as noted in the manual and forget it. Get a good gauge also. Digital is best bang for the buck...no need to spend hundreds.
 
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Rock&Roll

Rock&Roll

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the dash will say 34-36, and on warmer days or long drives the pressures will increase to 40 on the dash.

So this is the norm I'm thinking :headbang:

No worries if you read 34-36 and up to 40 when driving ?
 

Stripler

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My cold tire pressure with a digital and manual gauge is 32. My TPMS will normally also read 32 when I start the car. After driving a bit, the TPMS will usually read 35 sometimes 34. The recommended pressure (i.e., door jamb) is the cold tire pressure. It is normal for it to go up due to driving.
 

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Dspec_S550

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There is one more variable that I have not seen mentioned, and that is what your tires are filled with.

If you fill your tires with compressed air (atmosphere) there is water content, and that is why your tires pressure will fluctuate with temperature.

If you fill with nitrogen, your tire pressures will be more stable over a wider temperature range.
 

TexasRebel

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There is one more variable that I have not seen mentioned, and that is what your tires are filled with.

If you fill your tires with compressed air (atmosphere) there is water content, and that is why your tires pressure will fluctuate with temperature.

If you fill with nitrogen, your tire pressures will be more stable over a wider temperature range.
unless you're condensing the tiny amount of water vapor that remained in the air past the compressor (which would require sub-freezing temperatures), P1/T1 ~ P2/T2 will hold true in a closed system. Atmospheric air behaves like an ideal gas as much as N2 (it's 78% N2 anyway).

The most moisture you'll ever have in compressed air is when you take it directly from the compressor. If you take it from a cool storage tank the %MC will be minuscule. If you're airing up at any shop worth a stop it'll have a dryer on the compressor before the tank anyway.
 

scott_0

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OP, the PSI spec'd from Ford is cold, therefore take your readings cold and set them from there. its normal for the pressures to raise when the tires are heated (lol science and all) and Ive found my dash displayed pressure readings to be dead on with my high quality gauge
 

Dspec_S550

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unless you're condensing the tiny amount of water vapor that remained in the air past the compressor (which would require sub-freezing temperatures), P1/T1 ~ P2/T2 will hold true in a closed system. Atmospheric air behaves like an ideal gas as much as N2 (it's 78% N2 anyway).

The most moisture you'll ever have in compressed air is when you take it directly from the compressor. If you take it from a cool storage tank the %MC will be minuscule. If you're airing up at any shop worth a stop it'll have a dryer on the compressor before the tank anyway.
no big deal, but the "effect" on tire pressures from moisture is a bit more that miniscule.

Here is tire-rack's take on this, and I agree.

What are the effects of using pure nitrogen to inflate tires?
Nitrogen is a gas and is still affected by changes in ambient temperature (about one psi for every 10° Fahrenheit). Nitrogen filled tires will require pressure be added during the fall/winter months as ambient temperatures and tire pressures drop. Nitrogen is good but can't change the laws of physics.

Nitrogen reduces the loss of tire pressure due to permeation through rubber over time by about 1/3. This helps maintain the vehicle's required tire pressures a little longer, but doesn't eliminate the need for monthly tire pressure checks. This is good for people who don't maintain their vehicles well.

Nitrogen is a dry gas and will not support moisture that could contribute to corrosion of the tire's steel components (bead, sidewall reinforcement and belts) due to the absence of moisture over extended periods of time. However it's important to remember that atmospheric pressure is constantly pushing oxygen and moisture into the rubber from the outside of the tire. This is especially good for low mileage drivers who don't wear out their tires quickly or those that run average annual mileages but use long wearing radial (60K and 80K warranted) tires.

Nitrogen assures more consistent pressure increases due to increases in operating temperatures in a racing environment because of the absence of moisture. This is especially good for participants in track days, high-performance drivers education schools and road racing.
Drivers should use standard air if pressure adjustments are required when a local source of nitrogen can't be found during a trip. While this reduces the benefit of higher nitrogen content, it is far better than running the tires underinflated in search of a source. Often the original nitrogen provider will refill the tires for free or a nominal cost when the driver returns to his hometown.
 

F0J

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"It depends"

Pressure gauges are notoriously inaccurate. The environment can affect their readout. I bought a big ol' fancy gauge and even the expensive "lesser" gauges can read 4+ PSI off. One thing I noticed though is that once you've driven around a little in the car, the TPMS is pretty accurate.

Generally speaking, your fronts should have a bit more pressure due to the added load. The pressure also depends on a number of things like tire type, size, sidewall, driving style, speed, application etc.

For the PP summers, maybe 33 front, 31 rear?

I keep my RE-71Rs 265/45R18s 31 front, 29.5 rear. Which I think is a little low but it lets me put the heat in them a little quicker in the city. I aim for 36 hot.
 

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NightmareMoon

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Ford recommends 32psi cold, which will be something like 34-35 hot.

Once you've worn through a set of tires, you can fine tune. If your center of the tire wore through quicker, try starting at 31psi cold. If the edges wore out quicker, try 33psi.

Thats about all there is to it for a street car. For drag, track, or autox use there are other things to consider.
 

jbailer

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The TPMS doesn't have the most accurate of sensors, and temperature and barometric pressure certainly play a role.

I've found that when my dash says 32psi my gauges all say 28-30. When my gauges say 32 psi the dash will say 34-36, and on warmer days or long drives the pressures will increase to 40 on the dash.

The TPMS is a good quick & dirty way to see a problem, but it'd be better if there was just a green/yellow/red type indicator.
This isn't true. Many of the TMPS sensors are made by the same company, I believe it's Johnson Controls if not mistaken. They make the with an accuracy of +/1 1 psi. I used to calibrate pressure gauges (and a lot of other equipment) in the Army. Analog gauges are much less accurate first of all because of parallax error. Looking at the gauge at different angles, you could interpret the reading differently. Secondly because of the type of indicating mechanism they use like a Bourdon tube which aren't very accurate linearly. Digital gauges are much more accurate but still depend on the quality of the transducer. Obviously for the TPMS, they don't need a super accurate measurement but they are as good as almost any gauge you can find at an auto parts store.

Trust the readout on your dash unless you have a reason not to. You could verify your TPMS sensors once by taking a handheld gauge and checking the tire pressures of all you tires and compare them to the readout on the dash. You will find it's very consistent. As soon as I say that, someone is going to say theirs reads 5 pounds off. I'm not saying it's going to read the same, but it should be pretty consistent in that they would all read 4-6 pounds different. In that case, I'd say you have a bad handheld gauge. Now if 1 of them reads differently from the rest, you might have gotten a bad one. Some people think the computer is doing something, it isn't doing anything except displaying the transmitted value from the sensor. The TPMS sensor mounted in your wheels IS a tire gauge and a reasonably accurate digital one at that.

The reason the temperature changes as you drive or them sitting in the sun is because the tires are heating up. As the tires heat up, the air in them expands creating higher pressure. You don't need to compensate by changing the pressure you fill your tires to. The specified temperature is at the cold temperature (not driven). There's a lot of wiggle room there as the ambient temperature could be quite different. However, no difference. When it's generally hot, lets say it's 90* outside and you haven't driven, you have your tires at 32 psi. Driving on the road, your tires heat up to say 120*, your pressure will then read about 35 psi. A cold front moves in and temps drop to 50*, your pressure might drop to 28 psi. If it drops enough, you'll need to add air. All of that is expected and counted on with the 32 psi recommendation.
 

TexasRebel

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That's just it... you calibrated them. Not every model of pressure transducer (PT) is as accurate as the next... even the ones made by Johnson. Heck not even the ones within the same model line.

I used to calibrate differential PTs to measure the pressure drop across an orifice for USDA research. The curves varied wildly. We calibrated each PT individually after it was installed in its machine against a calibrated airflow meter. We related PT value to air speed in order to account for any manufacturing tolerances. I seriously doubt each TPMS PT is calibrated after the tire is installed. If anything, the tires are inflated to a pressure set by an external regulator and the vehicle is "taught" the PT value for each tire. (Even that seems like a lot for Ford to do) At this point the accuracy of the dash relies on the pressure regulator... while it will stay precise to +/-1 psi.

As the federal requirement for TPMS is to alert in the case of a 25% under-inflation... a few dollars can be (and likely is) saved by using sensors with higher uncertainty.

That said, you can't inflate based on the dash readout due to refresh methods and rates.
+/-1 psig from the [mass produced & probably has one size fits all calibration] TPMS sensors and +/- 3 psig on the [mass produced & who knows how it was calibrated] dial still fit the 2-4 psig low dash numbers I claimed earlier.

I mean... who wants to inflate tires while driving?
 

jbailer

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I seriously doubt each TPMS PT is calibrated after the tire is installed. If anything, the tires are inflated to a pressure set by an external regulator and the vehicle is "taught" the PT value for each tire. (Even that seems like a lot for Ford to do) At this point the accuracy of the dash relies on the pressure regulator... while it will stay precise to +/-1 psi.

As the federal requirement for TPMS is to alert in the case of a 25% under-inflation... a few dollars can be (and likely is) saved by using sensors with higher uncertainty.

That said, you can't inflate based on the dash readout due to refresh methods and rates.
+/-1 psig from the [mass produced & probably has one size fits all calibration] TPMS sensors and +/- 3 psig on the [mass produced & who knows how it was calibrated] dial still fit the 2-4 psig low dash numbers I claimed earlier.

I mean... who wants to inflate tires while driving?
Haha, so true. I don't use my dash readout to inflate my tires. My point is, they are an accurate indication of the pressure in your tires. There is nothing sophisticated happening there. The TPMS is a pressure transducer. It measures the pressure and transits the value to the computer which displays the value that was received. There's no math or calculations going on. Like you said, I'm certain they are not calibrated, that would raise the cost too much. It would only be checked as part of their QA cycle at manufacturing. I'd bet that as far as accuracy, they are checked and most accurate at 32 psi which is the most common tire pressure from manufacturers but who cares if it's inaccurate at 10 psi or 55 psi. I wouldn't be surprised like you said that they may be inaccurate across the range but my point is they can be relied on as a fairly accurate means of monitoring your tire pressure. I set my pressure with a cheap $10 gauge I bought at the AP store.
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