N/A tire pressure in the car

murphey

murphey

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murphey
The tire sticker on the door states 33 psi when cold. So what are reasonable psi in the various situations:

1) cold weather but driving on the interstate for an hour
2) warm weather, driving in town for over an hour
3) really hot summer, driving anywhere!
 
Always check and set your tires pressures BEFORE driving. That is the only way to get the right.

Conditions driving vary too much to say. And it depends on the weight of the vehicle also.

Also, define cold, warm, and really hot? Where I am sitting right now, the locals consider 70 to be cold. And I have been where it was over 120, so a bit hot.
 
murphey said:
The tire sticker on the door states 33 psi when cold. So what are reasonable psi in the various situations:

1) cold weather but driving on the interstate for an hour
2) warm weather, driving in town for over an hour
3) really hot summer, driving anywhere!
Click to expand...
You’d have to inflate to 33 psi cold, and then drive those scenarios to find out.
 
33 cold is way flat. I run 40 cold.
 
I don't have a good answer to your direct question, but I also wouldn't worry too much there. Basically, if your manufacturer gives you a cold tire spec, that provides enough margin for both 1) making sure that your tire pressures/temperatures don't get too high when it's hot out 2) maintaining good handling 3) whatever is considered an acceptable ride. But, something in the range of an additional 10 psi would probably be about right on hot days.

You even sometimes have manufacturers that will offer multiple recommended pressures, although that's rare. On the Jaguar XKR we used to have, there was a "comfort" tire pressure (26 all around I think) which had a speed limit associated of something like 120-130 MPH, and then a "performance" (which I think was around 32). The car drove better at the higher pressure setting so we kept it at that.

Tire temps can vary tremendously depending on what you're doing and with those pressure changes. Driving really aggressively (as in turns)? The temps can come up very quickly. When towing the Land Rover behind the RV, the left front tire (the one that is basically directly behind the exhaust pipe of the bus) runs about 10-15F warmer than than the others, which increases temperature.

Sac Arrow said:
33 cold is way flat. I run 40 cold.
Click to expand...

This really isn't a good answer, or at least a one size fits all one. For example, on my Land Rover Discovery the recommended pressures are something like 28F/32R. I tend to run them a bit higher than that while towing behind the RV for a slight reduction in rolling resistance and also the tires tend to leak a hair, so then I don't have to worry about adding air on a trip.

I put new tires on it a few weeks ago, and the tire shop completely ignored the sticker, set all the tires to 42 PSI. The thing drove terribly. Zero straight line stability, was darting all over. Basically at those pressures the tires were inflating to look like donuts and had a very small contact patch. Overinflation to an extreme like that will both hurt handling and then decrease tire life, since you're wearing all on the center of the tire.
 
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I generally set the tire pressures to what the door sticker says.
I set the tire pressure to 42 on the tires on our GT mustang because that is what the tire sidewall said and I was lazy and didn't want to get up and look at the door sticker.
It rode rough and stiff. Looked at door sticker and it said 32. So I lowered to 32 and it made it a much better ride.

I have done a lot of trailer towing, 26' 40' 49' enclosed race trailers and open car trailers.
I learned a long time ago to over fill trailer tires by 10psi over what the tire sidewall says. It keeps the tire cooler.

I also maintain a fleet of trucks since 1983. I just aired truck tires a few minutes ago as I am at work now.
Check out this service bulletin pertaining to trailer tires. 10psi over does no harm and helps a lot to prevent trailer tire blow outs at speeds above 65 mph. Who doesn't drive over 65 especially out west.

https://www.tirerack.com/images/tires/goodyear/Marathon_Special_Trailer_Applications.pdf

When out on the road with trailers, every morning there was pre trip inspection done without fail. That included checking all tire pressures when they are cold. It has saved me lot's of money and time. Cost me a few minutes of time every morning better than wasting time on the side of the highway changing tires not to mention the damage that occurs when a tire blows out.
 
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Gary Ward said:
I generally set the tire pressures to what the door sticker says.
I set the tire pressure to 42 on the tires on our GT mustang because that is what the tire sidewall said and I was lazy and didn't want to get up and look at the door sticker.
It rode rough and stiff. Looked at door sticker and it said 32. So I lowered to 32 and it made it a much better ride.

I have done a lot of trailer towing, 26' 40' 49' enclosed race trailers and open car trailers.
I learned a long time ago to over fill trailer tires by 10psi over what the tire sidewall says. It keeps the tire cooler.

I also maintain a fleet of trucks since 1983. I just aired truck tires a few minutes ago as I am at work now.
Check out this service bulletin pertaining to trailer tires. 10psi over does no harm and helps a lot to prevent trailer tire blow outs at speeds above 65 mph. Who doesn't drive over 65 especially out west.

https://www.tirerack.com/images/tires/goodyear/Marathon_Special_Trailer_Applications.pdf

When out on the road with trailers, every morning there was pre trip inspection done without fail. That included checking all tire pressures when they are cold. It has saved me lot's of money and time. Cost me a few minutes of time every morning better than wasting time on the side of the highway changing tires not to mention the damage that occurs when a tire blows out.
Click to expand...
I have a 1 ton dually pickup and tow a pretty heavy 5th wheel RV. I check cold frequently, sometimes just give the tires a kick to see if they all feel and sound about the same. I often will check with gauge when hot first stop of the day. I make them all the same then. Bleed some pounds of the hotter ones to match the coldest one. It’s usually a couple pounds. If one of them is more than that, I know I have a problem with that tire.
 
Sac Arrow said:
33 cold is way flat. I run 40 cold.
Click to expand...

Same here.

Higher pressure gives longer tire life, better handling, better braking, better gas mileage. Only downside is slightly rougher ride.

Manufacturer tire pressure has a large measure of comfort built in.

However, if the sticker shows a difference between front and rear pressures, unless you REALLY know what you are doing, keep that difference.

On road race cars, we check pressures coming off the track and then when cold. And adjust the cold pressures so that hot pressures are all the same. This is not really needed for street cars as conditions change to much to keep up with for a small increase in handling
 
My BMW will show what pressure to use and adjust depending on temp and how long you have been driving. So in the morning, it could show use 40, but when pulling into work, it might say, fill to 47. I dont know if the tire pressure sensors also transmit tire temp or if its just doing some linear maths.
 
Sac Arrow said:
33 cold is way flat. I run 40 cold.
Click to expand...
And if the inflation air is the same temp as the tire air?

Regardless of actual temperature, if the input air temperature is the same as the tire air, a given PSI will inflate to the same "stiffness" across the entire survivable temperature range.
 
My Tacoma’s door sticker recommends either 36 or 32 PSI, not sure which. The tires’ sidewall says 44 max. I keep them at 41-42. I tried 36 and the truck just feels a bit sloppy.
 
Don't forget that a large portion of tire pressure recommendations have to do with load capacity as well. Load capacity generally goes up with tire pressure. Plenty of tire blow outs on vehicles happen because the tires were underinflated for the load, which allows heat to build up more quickly and then it finally lets go. I run around in an 8K lbs SUV but keep my tires at 50psi because the ride is much nicer and it's still well within the load capacity at that pressure. I air them up to 60-65psi if I'm towing something of significance, but it can be punishing on a rough road if you don't have anything on the hitch. If you've ever ridden in a 90s dually with 75psi in the tires but nothing in the bed or being towed, it'll knock your fillings loose!
 
Lightly loaded Tundra, lots of curvy high speed highway - observations:

1. Sticker said 32 I think, I put them to 40… tread center section started to wear out, so I backed the pressure off.

2. usually people only notice the pressure in the fall when it gets cool, they look flat or the tpms gives a warning. But most tpms have no upper warning range.
This spring it went from 25 to 85°F in a week. My normally 35psi tires were now 41-42…. So don’t forget to check them in the spring as well.
 
SoonerAviator said:
Don't forget that a large portion of tire pressure recommendations have to do with load capacity as well. Load capacity generally goes up with tire pressure. Plenty of tire blow outs on vehicles happen because the tires were underinflated for the load, which allows heat to build up more quickly and then it finally lets go. I run around in an 8K lbs SUV but keep my tires at 50psi because the ride is much nicer and it's still well within the load capacity at that pressure. I air them up to 60-65psi if I'm towing something of significance, but it can be punishing on a rough road if you don't have anything on the hitch. If you've ever ridden in a 90s dually with 75psi in the tires but nothing in the bed or being towed, it'll knock your fillings loose!
Click to expand...

Totally agree, drove a 99 dually for everyday transportation for 130K miles for 13 years. It was easier when I was younger, as the years went by either the roads got rougher or I got older? lol
I still drive F250 everyday. Many days I wish I was in a 150 instead. But it sure is nice having the capacity to tow or haul heavy stuff.
 
With the tires cold (vehicle not driven), you should use the vehicle manufacturer's recommended pressure, assuming you're running the stock tire size.

The pressure shown on the tire sidewall is the maximum pressure approved by the tire manufacturer, not necessarily the appropriate pressure for any particular vehicle and/or loading.

If you have tire pressure sensors, you have to keep it set right or the vehicle will yell at you. Mine requires 33+/-10% or the light comes on.

With non standard tires, you have to experiment. I used to have a Jeep CJ5 with 33" tires for offroading; IIRC I needed something less than 15psi for street use. The usual approach was to draw a chalk line across the tire and then drive it for a few hundred feet. Proper pressure was when the chalk line wore evenly. Offroad, we'd air down to 5psi or less.
 
Gary Ward said:
I learned a long time ago to over fill trailer tires by 10psi over what the tire sidewall says. It keeps the tire cooler.
Click to expand...

I did the same thing and got longer tire life. Nothing like changing a tire on the side of the road in Texas in August...
 
cold is whatever the ambient temperature is that day..... just means the tires are ambient temperature and not warmed up from rolling

The tire manufacturers have standards that allow for whatever temperature and expansion pressure comes form normal driving... so pressure increases when you drive, but within reason the number doesn't matter. I think I've read something like 20%-25% over the cold pressure is a ballpark overpressure alarm point...but I don't recall what that is based on so it's worth what you paid

I have an aftermarket tire pressure and temperature monitoring system with little sensors that replace the dust caps on my motorhome's tires. The unit alarms for over or under pressure, and for over temperature. I don't recall what my over alarm set points are based on but it's some arbitrary percentage. It has always been fine around here. Several years ago took a trip through the desert out to the Grand Canyon, and in teh heat of the day I was getting alarms a lot. Caused a lot of needless stress and worry.

the standardization body for these things
https://www.us-tra.org/aboutTRA.html
 
Sac Arrow said:
33 cold is way flat. I run 40 cold.
Click to expand...
Depends on the vehicle and the tires. I figure I don’t know more about the engineering bits than the people who designed the car (or truck), the people who developed the tires, and the people who did the testing. I’ll take their advice for daily driving.

That said, if I’m at the track it’s an entirely different story. In that environment, I’m the test pilot - so to speak.
 
Brad W said:
I have an aftermarket tire pressure and temperature monitoring system with little sensors that replace the dust caps on my motorhome's tires. The unit alarms for over or under pressure, and for over temperature. I don't recall what my over alarm set points are based on but it's some arbitrary percentage. It has always been fine around here. Several years ago took a trip through the desert out to the Grand Canyon, and in teh heat of the day I was getting alarms a lot. Caused a lot of needless stress and worry.

the standardization body for these things
https://www.us-tra.org/aboutTRA.html
Click to expand...

lol
Reminds me of a engine monitor for 0-320 C172...I don't have one for my 172.
I have also trailered through the Grand Canyon and Vegas at the end of July. Hotter than hell I am betting and luckily I made it out of there with my tires intact without a tire monitoring system. Sometime less is more...
 
Gary Ward said:
Totally agree, drove a 99 dually for everyday transportation for 130K miles for 13 years. It was easier when I was younger, as the years went by either the roads got rougher or I got older? lol
I still drive F250 everyday. Many days I wish I was in a 150 instead. But it sure is nice having the capacity to tow or haul heavy stuff.
Click to expand...

The Excursion doesn't ride like an Expedition/Tahoe, lol. You hit a pothole, you're going to feel it. If you've got 65psi in the tires and hit that pothole, you're also going to consider making a chiropractor appointment. The front leaf springs only have like 2" of travel from the factory, so that's where most of the problem lies in my situation. You can convert to the coil springs and the ride gets way better, but then it raises the ride height and it won't fit in my garage anymore. 1st world problems.
 
Zeldman said:
I did the same thing and got longer tire life. Nothing like changing a tire on the side of the road in Texas in August...
Click to expand...

In approx 200K miles of towing, I have had only 3 tire failures. First one was one of my drive tires on our RV and was noticed by the guys washing it out west somewhere. I drove across the street from the truck wash to a truck service and they fixed the tire.
Second time was in Fla and was a trailer tire. State trouper pulled up behind me with his lights on to help keep us safe. I had all the tools, spare and jack to help our self's.
The third time was after 2000 miles and 5 miles from home just as I slowed to get off the interstate. It was a huge explosion and blew the fender just about off of a open car trailer. I unloaded the car off the trailer and asked my wife to drive it the rest of the way home. I took the blown tire off and towed the trailer the rest of the way home on 3 wheels. 2+ weeks on the road and 5 miles from home and all hell let loose.
 
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SoonerAviator said:
MOST of tire blow outs on vehicles happen because the tires were underinflated for the load, which allows heat to build up more quickly and then it finally lets go.
Click to expand...

FTFY.

Number one cause of tire failure is low tire pressure. That is why cars are mandated now to have tire pressure sensor systems.
 
SoonerAviator said:
Don't forget that a large portion of tire pressure recommendations have to do with load capacity as well. Load capacity generally goes up with tire pressure. Plenty of tire blow outs on vehicles happen because the tires were underinflated for the load, which allows heat to build up more quickly and then it finally lets go. I run around in an 8K lbs SUV but keep my tires at 50psi because the ride is much nicer and it's still well within the load capacity at that pressure. I air them up to 60-65psi if I'm towing something of significance, but it can be punishing on a rough road if you don't have anything on the hitch. If you've ever ridden in a 90s dually with 75psi in the tires but nothing in the bed or being towed, it'll knock your fillings loose!
Click to expand...
+1. Door sticker pressure assumes you are using tires of the same load range that the vehicle was originally sold with. My F150 door sticker calls for 35 psi, but that pressure would be entirely incorrect (and dangerous) to use on my truck given that I have heavier load range LT tires on it now. In fact, the heavier load range LT tires actually have a lower weight rating at 35 psi than the original tires had.

I typically run tires at whatever the max pressure rating is on the tire sidewall. In the case of my F150, with the LT tires, that is now 80 psi. I could certainly get away with 50-60 psi and get a little gentler ride, but I don't think the safety tradeoffs are worth the ride difference. The higher the pressure, the lower the tire temperature at higher weights. Additionally, NASA testing indicates tires generally hydroplane at 8.6 times the square root of the tire pressure. The higher your tire pressure, the more hydroplane resistance. I hate hydroplaning. 80 psi puts the dynamic hydroplane speed up near 76 mph vs. 50 mph at 35 psi.

A lot of this depends on what you drive & how you drive it.
 
Sac Arrow said:
33 cold is way flat. I run 40 cold.
Click to expand...
That's your car/truck/whatever. This is a car. Smaller tires, smaller gross weight.
Ted said:
I don't have a good answer to your direct question, but I also wouldn't worry too much there. Basically, if your manufacturer gives you a cold tire spec, that provides enough margin for both 1) making sure that your tire pressures/temperatures don't get too high when it's hot out 2) maintaining good handling 3) whatever is considered an acceptable ride. But, something in the range of an additional 10 psi would probably be about right on hot days.

[snip]

This really isn't a good answer, or at least a one size fits all one. For example, on my Land Rover Discovery the recommended pressures are something like 28F/32R. I tend to run them a bit higher than that while towing behind the RV for a slight reduction in rolling resistance and also the tires tend to leak a hair, so then I don't have to worry about adding air on a trip.

I put new tires on it a few weeks ago, and the tire shop completely ignored the sticker, set all the tires to 42 PSI. The thing drove terribly. Zero straight line stability, was darting all over. Basically at those pressures the tires were inflating to look like donuts and had a very small contact patch. Overinflation to an extreme like that will both hurt handling and then decrease tire life, since you're wearing all on the center of the tire.
Click to expand...

This is what prompt my original question. Sticker says 33 psi. Shop (cold tires) put 40 psi in. This is a small Ford SUV, never runs at gross weight, I kept getting the tire pressure alarm. Stopped after about 30 min and brought the pressure down to 38 psi (from 44 on the road). These days the numbers are 34 cold to 39 on the highway. I really don't need a blowout on the road...BTDT once.
 
murphey said:
This is what prompt my original question. Sticker says 33 psi. Shop (cold tires) put 40 psi in. This is a small Ford SUV, never runs at gross weight, I kept getting the tire pressure alarm. Stopped after about 30 min and brought the pressure down to 38 psi (from 44 on the road). These days the numbers are 34 cold to 39 on the highway. I really don't need a blowout on the road...BTDT once.
Click to expand...

That won’t cause a blowout, but that’s the tire shop being dumb.
 
Speaking of blowouts...

I was getting ready for a trip with my first 5th wheel. During the day before leaving, I was in and out of it several times preparing for the trip. The middle of the day, I exited the trailer and was about 3 feet away when the right front tire let go. I mean it LET LOOSE. Loudly. I mean almost soiled my trousers loud....

All this while parked in my driveway.

So, while on the trip, I come to a little one red light town. Of course the red light stopped me. There were 3 kids on the corner, about 10 years old. While waiting for the green, I hear a BOOMP come from behind me. I look in the right rear view mirror and the kids are hauling butt down the road. I am thinking those little excrements threw something on the side of my trailer. About an hour later I stop for lunch, and to look at what the kids threw on my trailer. Nothing. I don't see a thing. Oh well.

About a month later I have my truck up on the rack to change the oil. As I am greasing the U-joints, I see a tire gator laying across the rear axle. I grab it to get it off the axle, but it is stuck to something. and stuck it is, to the rest of my spare tire. Seems like the spare tire blew out while I was at the red light. So that is why the kids were running away.

I guess I was lucky the kids didn't return fire...

The moral of the story.??

I don't buy used tires anymore.
 
Pinecone said:
FTFY.

Number one cause of tire failure is low tire pressure. That is why cars are mandated now to have tire pressure sensor systems.
Click to expand...
That, and the fact that half of drivers these days wouldn't know how to check the tire pressure if their life depended on it.
 
I inflate to the door jam sticker values, never had a problem. I check all tires (and fluids, etc.) once a month and before any long trips.
 
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About a month ago I was airing up a 19.5" recap on the garage floor. I was using a clip on air chuck while I did something else in the shop like I usually do.
First I hear this metal like flex sound of the tire filling up. I knew something is wrong, just then the tire explodes out of the sidewall and flung itself 4-5 feet in the air. Sounded like shotgun, it blew out the sidewall of a re cap tire. I didn't take time to read the date code on the casing but I estimate it being about 10 years old, the casing. The recap tire was only on the truck about 2 years. It was old F450 that we use to haul scrap metal. So it does not get driven everyday. Close call and loud but it did not get me. It blew up because the casing was too old.

I used to mount and dismount split rims in the 80s and never had one blow up. Didn't use a cage either back in the day.
These tires are the easiest to mount and dismount and maybe the safest?
MVC-024S-2.jpg

I polish them also.
039-1.jpg
 
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I have another funny tire air pressure story.
I 1986 I had 4X4 ramcharger with 33" tires on it.
My mentor was a jeep guy mechanic and was part of club that held sand drags and hill climbs on pea gravel actually. So few years I entered my ramcharger to drag race it. The standard was to run really low air pressure which I am sure most know on sand etc.
Around 2014 I get my wifes durango stuck in the sand out on the outer banks where we were allow to drive with a off road permit. After digging and trying a bunch of things I called a tow truck. Guy came out and winched the thing about 12" forward out of the hole I was in. I am like no way, you got to pull me farther than that. He starts to let air out of all 4 of our tires!! I remembered just then about my sand racing days. Felt pretty stupid as the tow truck driver drove our durango all around staying on top of the sand showing the way to drive out there. Great guy and I tipped him. He told me where to air up just as you come off the sand.
Doh forgot about that trick.
 
Gary Ward said:
I used to mount and dismount split rims in the 80s and never had one blow up. Didn't use a cage either back in the day.
Click to expand...

I did split rims as well. I never blew one up but someone else did. And we did use the cage. After that I had a whole new respect for split rims. I still break out in cold sweat when I see one of the cages at old abandoned truck stops...

images
 
I didn’t take the time to read all of the thread,
But to the OP, if you didn’t set recommended tire pressure when they are cold and want to check air after driving you’ll see readings higher (I’ve seen up to 7 psi higher) than what they were when cold. But adding cold air to the mix might dork up any estimate of a warm tire. What is interesting is seeing how something as simple as the sunlight on one side of a parked car can increase that side’s tires by several PSI before driving. As far as if you should use the recommended setting or something else I’ll stay out of that can of worms.
 
Chemical engineer side of me:
Air, nitrogen, and carbon dioxide at passenger vehicle tire pressures all follow the ideal gas law pretty well, so you can estimate pressure change by knowing the temperature change according to the formula:
Pressure * Volume = Number of molecules * R (ideal gas constant) * Temperature

Assuming that the volume is constant, and the tire isn't leaking (number of molecules within it is constant) this leads to Delta P is proportional to Delta T. You just have to convert temperature to degrees Kelvin (and to be clear, this is the temperature of the air within the tire which can be significantly above ambient temp).
Re-arrange the ideal gas law equation to: P/T=N*R/V
Then since the right side of that equation is assumed to be constant, re write to P1/T1 = P2/T2
Re-arrange it again to find P2 = P1*T2/T1

To give an example, P1=35 psi, T1=60 f, T2=90 f P2=?
convert temperatures to kelvin: T1=288.7 k, T2=305.4 k
P2=P1*T2/T1 = 35*305.4/288.7 = 37 psi

2+ decades of developing the ride, steering, handling, ESC/ABS/TC, and tire development side of me:
Set the tires to the placard pressure in the driver's door jam with the tires at ambient temperature and no sun load and don't worry about it (check them the same way every couple of weeks if you don't have TPMS) unless you are doing something like racing, then you will want to be more watchful and play with the pressures to get the car to feel right.
 
Larry in TN said:
How could it be?

When you pressurize the air, that air's temperature increases significantly.
Click to expand...

I usually fill my tires from a portable tank, that I might have filled months ago, so the air in it is ambient. If I haven’t driven the car recently, so is the air in the tires. In that case, the higher pressure air leaving the tank cools as it enters the tire
 
GaryM said:
I usually fill my tires from a portable tank, that I might have filled months ago, so the air in it is ambient. If I haven’t driven the car recently, so is the air in the tires. In that case, the higher pressure air leaving the tank cools as it enters the tire
Click to expand...
But the air that is already in the tire gets compressed, so it heats up. If you use a sensitive enough gauge, you can watch the pressure go up as you fill the tire then drop as the two air masses equilibrate.
 
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