Car Tire Inflation


I’m an engineer, and ambient temperature is whatever the temperature is outside. It could be -20 or +120, it doesn’t matter. Ambient refers to the instantaneous temperature at any given time. It changes throughout the year and even the day.


I don’t know what industry you work in, but I have never met an engineer that defined ambient as anything but STP.

Edit: but every engineer I worked with never wanted to leave the lab.


I work in aeorospace, and ambient is whatever the prevailing temperature is at the time. It is often understood to be 20C in the test lab because the lab is a controlled atmosphere. Ambient temperature on the runway is occasionally 20C, but is almost always something else. Ambient temperature in space is way different, yet the term is still used.


This is a service bulletin to help adjust tire pressure for outside temperatures when inflating tires at room temperature;

“Cold” tire pressure, as shown on the tire pressure label on our vehicles, is generally considered to
be the pressure in a tire that has not been driven in the past 4 hours and has been parked outdoors.


The purpose of this chart is not to adjust the pressure for what is best for the tire but to prevent the TPWS light from coming on. I found this same chart in my daughters owners manual packet for her 2014 Camry.


Correct, the TPWS light will illuminate if the tires are under inflated, the chart shows how to compensate for outdoor temperatures so the tires will be properly inflated.


OK guys, I have made one error. ambient is the actual temperature. What I should have said is that when engineers do their thing, they assume ambient as STP (standard temperature and pressure) and I’ve seen two different STP’s used. Pressure is always 960mm but temperature is either 0C or 20C. Everything they test uses that as their baseline.

My question is, have they ever actually tested at other temperatures?

The reason I ask this question is that I am old school Vermont. In the winter, people used less pressure in their tires (not too much less) so they would heat up more and get better traction. Keep in mind that they didn’t drive nearly as fast in winter as they did in summer. They couldn’t go too low on pressure or the tires got less traction. The trick was to find the optimum winter pressure for the best traction.


Standard pressure at sea level is 760-mm of Mercury, not 960-mm.

Yes, pressure testing at different temperatures has been done for a long, long time. So long, that it typically isn’t repeated unless a Deflategate situation arises.


My bad, fat finger that one didn’t I.

How do you know that testing was done at different temperatures?


An equation was developed and has been proven many times. So often that it is referred to as Gay-Lussac’s Law.


Another reason-using STP woiuld require both a pressure gauge and a thermometer, along with a correction table. Tires are designed for a given pressure, regardless of temperature.

Can you point to any reference that supports your opinion?


Lets see, door jam label has tire pressure numbers. That is what the tire shop does when I stop by to have tires checked. Summer or winter. Why make it complicated?


What does that have to do with the performance of rubber at different temperatures?


You asked how I know that testing was done at different temperatures and I answered that question. It did not appear to me thatmeasurign pressure at different temperatures has anything to do with rubber.


I’m a little confused by the point that keith is trying to make. So if this is a bit long, I apologize.

Tires are typically tested for durability in the lab at 100°F. Why that temp? Because it’s a nice round number and it is a common temperature for the hot part of the US and people can actually work in a room at that temp (at least for a little while!)

  • BUT -

There are a lot of lab tests that are conducted at the ambient temperature of the lab - and that’s typically 72°F. They do that because the temperature is not critical to whatever property is being tested - and even if it is, the test results will be reported at that temperature and whoever is looking at the test result will have to adjust for whatever temperature they are interested in.

So what lab tests are conducted at room temp? Rolling Resistance, Force and Moment, Plunger Energy, etc.

What about lab traction testing? The traction of a tire is too large to be conducted on present lab machines - so traction is tested outside at the proving grounds.

So what about outside testing? That’s done at ambient as well - and the results are temperature corrected based on past results.

What past results? The ones conducted on SRTT’s (Standard Reference Test Tires), which if I remember correctly is an old design Uniroyal tire that Michelin produces periodically (under very tight tolerances), stores in a salt mine somewhere, and tests to make sure the latest batch is comparable to earlier batches.

It is common for outdoor tire test procedures to call for a run using the SRTT as a control. Traction testing is one of those.

HOWEVER, subjective ride and handling tests (also conducted at the proving grounds) are done without the use of an SRTT. Those tend to use a target tire - something current and appropriate. There may be some temperature extremes where these tests are not performed, but I don’t know that for a fact.

And the exception here is snow traction testing which is done at ambient as well, except it has to be cold enough for there to be snow. Again, there is a control tire, typically the SRTT.

There are some wear and durability tests that take place at the proving grounds. These are also conducted at ambient temperatures. For example, the UTQG Treadwear test is conducted using the SRTT on a prescribed course in Texas, and I think there are temperature limitations for it. (They also can’t run the test when it rains too much)

So, No! STP (Standard Temperature and Pressure) is hardy ever used when it comes to tire testing.

Lastly, as far as the old school Vermonters lowering the tire pressure in winter. I’ll bet that is all about traction as a larger footprint gives better snow traction. It doesn’t help the tire penetrate through the snow to the pavement where the traction is much, much better, but lots of folks ignore that in favor of getting unstuck from a snow bank.

So that was a lot of words. I hope that somewhere in all of that, the concerns were addressed. If not - ask!


As I read it, Keith wants folks to adjust the inflation pressure away from ‘recommended’ based on the outside temperature. Not appropriate in my opinion.


Actually no. What I am really trying to get at is if you adjusted your tire pressure on a 60 degree day and the temp drops to 20 a couple of days later, it is not necessary to go out and readjust your air pressure. You can leave it alone, it won’t hurt anything.

I check and adjust my tire pressures about every three months, and I am a bit anal about it. I use an IR thermometer to determine tire temperature, especially in the summer where it can be 80-90, though I try to do this early in the morning then when it is cooler and I adjust the target pressure. Then I add three pound to this as the tires seem to lose about a pound a month.

I get very good tire life, often over 100k miles from a set. I rarely have any tire problems except lately, I hit a rock that tore up the sidewall on one of the tires on my Subaru. The tires had about 56k miles on them, the tread depth was 6/32" when this happened. Bought four new tires and a month later, hit a box of nails that someone dropped in the road. Both front tires replaced, the rears were OK as the front tires absorbed all the nails in the path. 32 nails in one front tire.

Around here single digits are about the lowest temps we ever see, usually low 20’s is the normal coldest seasonal temperature. 100+ occasionally in the summer.

I am also anal about alignment as well, but I rarely allow anyone to realign my vehicles. Only if I see an unusual wear pattern. I found out many years ago that most alignment shops will make a good alignment bad and a slightly bad alignment worse. If you see how the alignment is done in the factory, you would not change it without a very good reason.


That’s what I said. You want folks to use the cold temp pressure as the recommended pressure, instead of what the sticker says. That’s incorrect.


Huh ???


In this case, your tires will be about 4 PSI too low, which in my opinion is too much.