Tire pressure cold vs hot

We just got back from a 4000 mile trip with the new TPMS, and I was shocked at the variation in pressure from cold to running.

Before leaving Houston, I carefully set the pressures to 65 psi all-around, on a typical 85-degree July morning in Houston.  While underway, the temps varied considerably, with the rears running 10 degrees hotter.  The hottest was the right rear, which I figured must be because the exhaust runs close to that side.  Pressures were all different and all over 70 running, with the rears approaching 75.

But then, on one 50 degree morning in Yellowstone, the low pressure alarm went off as soon as the sensors started to come up, as I was leaving the Fishing Bridge RV park.  One of the rears was below the 55 psi low-limit I'd set.  Oh, well, I figured, this was a chance to actually set the pressure "cold".  So I pulled into the Fishing Bridge service station and carefully adjusted all six tires to 65 psi.  After they warmed up, I was a little concerned to see them all running over 75 psi, but figured it must be normal, since 50 degrees is just a typical cool morning anywhere but Houston.

A couple of days later, and much further South, the HIGH pressure alarm went off when one of the rears hit the 80 psi limit, and the temp on that tire hit 117 (on a 100 degree day in Colorado).  So at the next fuel stop, I released enough air to bring them all back down to 75.  Funny thing was, the hot side was on the left this time.  So I guess it has more to do with the wind direction than the exhaust.

I'm tempted to just set the alarms for a ridiculous wide range and forget about it.  I never used to worry about this stuff when I couldn't see what it was doing!  But I sure never imagined that huge truck tires like these could vary 15 psi from cold to hot!

I guess this is normal?  By the way, it's the EEZTire TPMS, which seemed to work fine.

Your swings are significant, but I wouldn't know if that much is typical.  The only way to stabilize your tire pressure through all the extremes is to replace your air with nitrogen.  The people who do it for you, add those green valve stem caps to indicate so.  I believe most tire stores offer nitrogen service....best to call ahead for availability and pricing.

Air pressure varies with temperature and altitude. The air in a tire does heat up as the tire flexes, so pressure rises. As you gain altitude driving from Texas to Yellowstone, pressure rises. Our Tire Minder TPMS behaves the same as yours but I don't worry too much about temperature since the sensor is in the valve cap which is outside the tire and probably not very accurate. No matter how carefully I set the pressures, the TPMS always reports various other pressures. I have had low and high pressure alarms when in fact the pressure was what I had set it to, so I don't think the pressure reading is very accurate either. Still it's nice to have a warning when the pressure is really wrong. I can live with the false positives.

Another factor that can make your tire pressures and temperatures vary considerably is if one side is exposed to direct sunlight and the opposite side is completely shaded.  We have also had our TPMS temperature alarm go off when we are pulling off at a rest area or other stop because of the lack of air passing over the tires which then causes the tire temps to spike.  It is not uncommon for the inside dual to run hotter than the outside tire due to the amount of air that is able to pass over the tires.  Even with these issues, having a TPMS is well worth it to alert you to a developing problem as you are driving down the road.
Gary

Set it in the morning cold and don’t worry. Set the temp alarm to 157-160. On a sunny 87 degree day asphalt heats up to 143 degrees. Add your tires rotation and veh weight to the mix and some high temps can be seen quickly. The temps your tires reached are not out of line Imo.

I don't have a TPS system.   On a trip traveling about 500 miles in one day from Oklahoma City to Raton, New Mexico (which we do a couple of times a year in the summer), I lower the pressure in all tires by about 7 or 8 psi.  I know in going down hot highways in Oklahoma and Texas in the middle of summer my I will pickup that or more in psi.   

I check our tire pressure frequently while traveling and I do two tests:  psi and temperature of each tire.  I have a digital laser thermometer I bought several years ago from Sears for twenty bucks on sale (back when there was a Sears    ).   Internal temps on a specific tire will rise if the tire is having certain problems. 

Paul

I have no TPMS (yet) but I check the tire pressure every day.  We are usually on the road early but one morning we got up late.  The coach was parked North - South so one side was exposed to morning sun and the other in the shade.  Shady side was 78psi about where it should be but the sunny side was 87psi.  Just sitting still, and all tires had been equal the day before.

I was going to let air out of the sunny side tires but elected not to assuming that once we started moving things would equalize.

"Keep quiet and be thought a fool, open mouth and remove all doubt"

I am one of those guys who has two left hands and hardly knows how to hold a screw driver.  My best tools are pen and checkbook.  With all that said, I feel that tires are engineered and manufactured to handle most anything the road can dish out.  My final thought... if the tires are properly inflated when cold, then all is good.  Oh, nails and such can foil that idea.  Include me in the group that has never had a TPMS.

Cheers...
Barry T

Before leaving Houston, I carefully set the pressures to 65 psi all-around, on a typical 85-degree July morning in Houston.  While underway, the temps varied considerably, with the rears running 10 degrees hotter.  The hottest was the right rear, which I figured must be because the exhaust runs close to that side.  Pressures were all different and all over 70 running, with the rears approaching 75.

But then, on one 50 degree morning in Yellowstone, the low pressure alarm went off as soon as the sensors started to come up, as I was leaving the Fishing Bridge RV park.  One of the rears was below the 55 psi low-limit I'd set.

Pressure is directly proportional to absolute temperature. PV=MRT  Boyles Law

so 85oF = 545oR
    50oF = 510oR

Multiply (65 psig ) X 510/545 =  60 psia
5000'   altitude the air pressure is 2.5 psi lower
10000' altitude the air pressure is 4.6 psi lower

So your experience is what would be expected.

All tire manufactures say check pressure when tires are cool and leave alone.

Where would I find the recommended tire pressures for a 2018 2910D?  Totally forgot to ask about this when we picked it up!

I think inside your sink door is a label.
Go to Michelin's website, no matter whose tire brand you have, they have a chart for RV tires, weight vs pressure. To really do it right you need to weight each axel and each side. while fully loaded. Then highest load on one axle sets what pressure you need on that axle. Same for second axle.

I think inside your sink door is a label.
Go to Michelin's website, no matter whose tire brand you have, they have a chart for RV tires, weight vs pressure. To really do it right you need to weight each axel and each side. while fully loaded. Then highest load on one axle sets what pressure you need on that axle. Same for second axle.

Link - https://www.michelinrvtires.com/reference-materials/load-and-inflation-tables/#/



Motorhomes and other post-factory finished vehicles have (since 2000 or so..) a sticker provided in the driver's (left) door frame with recommended tire pressures for that finished vehicle. This is provided by the finishing manufacturer, not the original equipment maker.  I just checked our 2 'cube' vans and one 'plumbers' van, and they all say 80 PSI. All have LT245/75R16 E tires, but GVW for the cube vans is 13,500 and the plumbers van is 11,500.

On this sample size of (3) vehicles, I am guessing that the rule is probably 'fill to max pressure stated on the tire for the single/dual application'.  Very scientific.

Factory finished vehicles have a sticker provided by the OEM, being that their designed use limits are known when they leave the factory.

RE: wider pressure variations than explained by the altitude/temperature changes.  Tire moisture is a HUGE variable to consider.  Part of the nitrogen tire filling benefit is the lower permeability and decreased expansion/contraction with temperature. IMO, one of the biggest benefits is the removal of the oxygen also removes ALL water. (OK, not *all*, but down to .01 PPM. Close enough for me.) 

Here in humid Florida, I have demounted tires that were pumped up with the 'side of the station' stand alone emergency air compressors. Usually several times, as owners had to find time to come in and get it repaired. Not uncommon to find quite a bit of water in there.
When I HAVE to use those emergency compressors, the first thing I do is 'waste' a bit of the air blowing out the line and chuck. I have been 'rewarded' with a stream of water out of some. Likewise for the service station 'island' that has air supplied by the shop, run underground to the island. The cool ground condenses moisture, and it can come streaming out. Most people never check, and it goes directly into the tire. Along with any crud that is lodged in the chuck the last person threw on the ground...

My opinion for trucks is to run the tires at the maximum cold inflation pressure shown on the sidewall.  Before getting any more particular about exact pressures, who here has had their gauge calibrated on the Snap-On truck or other location? I did!   
We get these type of pressure gauges for shop use -

for most of the equipment using 50PSI or less. Taking 5 new ones out, and checking the SAME tire, they were +/- 3 PSI out of the box. ~5% error.  IE: a tire at 35 PSI read anywhere from 32-38. The results were repeatable, so the gauges were consistent, but not calibrated. But they are close enough and cheap enough for our use.
My 'Way too expensive' ($50+) gauge is <1%, and used to be checked yearly. My new shop is no longer serviced by Snap-On, so the gauge has not been calibrated since ~2014. I got it in 2001, and it never needed adjustment, so I feel safe in saying it is probably on spec. It does have its own case, and lives there unless it is being used.

Do I have nitrogen in my tires? Nope. I keep them at the maximum sidewall pressure at 'room temperature' which is pretty easy to keep near here in FL.  Everything I have heard or learned says NEVER adjust tire pressure when hot... so I do not. So far so good... 

I posted a warning about tires gauges.  You can read of my at-home research by CLICKING HERE.  My findings were quite alarming.

I posted a warning about tires gauges.  You can read of my at-home research by CLICKING HERE.  My results were quite alarming.

Once again...well done sir!   

I posted a warning about tires gauges.  You can read of my at-home research by CLICKING HERE.  My results were quite alarming.

Interesting analysis.  The thought occurs to me -- how accurate can a tiny sensor hung on the valve stem of a moving vehicle be?  I think this confirms that the thing to do with a TPMS is to set it to a ridiculously wide alarm range and don't worry about it.

I might just order one of those Accutire gauges.  I use a "Slime" gauge myself and agree about the annoying complexity.  Mine just has one button, which you have to constantly punch because it keeps going to sleep.  But every time you hit it, it thinks up a new unit of measurement to baffle you with.