Your logic is absolutely flawed and ill advised. The tire will heat up during drive cycle, regardless of ambient temperature. When the ambient temperature is colder say it dropped 20degF, you lost 2 psi, how in the hell would you want to adjust the tire pressure even lower? The same goes to higher ambient temperature, say goes up 20degF, your tire gained 2psi, then you tell people to adjust the tire pressure even highter? That is dumb.

 

You're adjusting to achieve a desired pressure at a reference temperature. If you want to always have the same pressure at 68F, then as you pointed out, at 58F that pressure will be lower.

So if you want 33 psi at 68F but your garage is 58F, what pressure should you put in so that if your garage warmed up to 68F you would still have 33 psi? Certainly not 33 psi. And certainly not more than 33 psi.

Science tells us clearly that psi will drop .1 for each degree F of cooling. Or rise .1 psi for each degree F of rise. That's 1 psi per 10F. So if you fill your tires to 32 psi when your garage is 58F and over the course of the day your garage warms up to 68F (your reference temperature) then your tires will be at the desired 33 psi.

Anything other than adjusting your fill pressure down for coffee ambient will result in undesired pressure. The same holds true in the opposite directing if you have higher temperatures. So if your garage is 78F you would want to fill to 34 psi.


OP ... Did I tell you that you opened a can of worms?!?!?!

Edit ...

Further to the above, and a different view, more favorable to E39Nutz ...

My side of this was ... If you want to always have the same pressure at 68F

Is this a correct premise?

The debate is whether we are adjusting our pressures specifically to the current ambient temperature in which case we add air to achieve desired pressure when cold temperatures prevail and release air when the weather gets warm, or if we are chasing after the desired pressure after we've reached a given specified ambient? Which method is right? There's no doubt that cold weather can upset the TPMS and that favors adding air when cold so that the desired pressure is met regardless of any reference temperature.

Many years ago, living in Phoenix and owning only a motorcycle, I rode to work pre dawn with snow flurries knowing the heat coming off the pavement would be triple digits on the afternoon ride home. That would be an 8 psi change. How would you set your tire pressures for this? I set pressures when the temperature was somewhere near the middle. This worked on the street but might not fly on the track.

But many of us live in conditions that create wide ranges in ambient temperature throughout the day. If this is the case, are we going to get out of the car every 5 minutes to adjust pressure? Or release air on the sunny side of the car and add air on the shady side?

Adias suggested to me a (T1+T2)/2 method to use average ambient and adjust pressure accordingly. I interpret this as for a given driving period, T1 is lowest temperature while T2 is the highest. This adjusts for fluctuations. And worked for me in Phoenix.

Depending upon who you follow ...

http://www.tireprofiles.com/adjustin...nt-conditions/

https://m.tirerack.com/tires/tiretec....jsp?techid=73

As is pointed out, it is not the tires that support our car, but the air in the tires. Are we interested in the support of the air molecules, or the pressure they exert?

Looking up pressures vs temperature, a lot of people refer to ambient as the current conditions and assume the recommended cold tire pressure is per this variable value. This seems to be favored.

These days I look at my TPMS display in the morning and adjust my pressures accordingly when necessary, remaining fairly oblivious to what the ambient is or isn't. I'd rather be over inflated than under. While these days I run 33F and 38R, when I was commuting 60 miles a day I ran 38F and 44R. So what can I say?

Fortunately there seems to be a wide range of "acceptable" pressures available to us.

Now who wants to start another oil thread???

 

Can the ideal gas law help decide this?

(P1 * V1) / T1 = (P2 * V2) / T2

 

Yeah, that makes no sense - if the running temperature of the tire hit the same level regardless then sure, but it stands to reason that a cold morning is likely to have a cooler midday temperature too. If your tires always hit 100 degrees during a drive regardless of the morning temperature (and maybe that's the case in the desert where there can be massive temperature swings), then start with the lower pressure. But if your morning is cold because your day will be cool, then you need to increase pressure to make sure your cool-running tires aren't underinflated. The 997.2 tire pressure gauge seems pretty accurate, watching to see max temperatures on it might help define where you start in different conditions.

But don't underinflate on a day when the tire is likely to run soft...

 

As a kid working my way thru college, I worked for a large tire sales/installer and was always taught to look at this a little differently. The reason you set your air pressure at a given pressure and temperature such as 33 lbs at 65 degrees is to arrive at the correct or ideal pressure once the tires have been driven 5-10 miles and have reached full operating temperature. That ideal pressure might be 37-38 lbs. Using the universal gas law principle, at 40 degrees you would set your pressure to 30-31 lbs to allow for the temperature difference. However if you do, you will never reach the ideal pressure of 37-38 lbs when the tires are fully heated and you will run under inflated. Accordingly you should set your cold pressure to 33 lbs at 40 degrees in order to reach that ideal temp of 37-38 lbs when the tires are fully heated. This should be easy to check since all newer cars have TPMS where you can see tire pressures both cold and when when fully warm after driving a while.

Then again, I did that work many, many years ago so the whole thing might be an old wives tale at this point.

 

"But since it's molecules that support the car, it's the number of molecules you pump into the tire that you are concerned with."

Molecules do different work based on their energy state, that's where PV=nRT comes from. I could care less about the number of molecules in my tire, I care only about how hard they are pushing against the tire! Look at your math - the same number of molecules at 0 and at 100 have dramatically different effects.

 

Good points. I agree with you.

 

Good points made by many here. Please allow me to eat some humble pie.

Points made by others negate the idea of chasing a desired psi at a specific temperature. My foolishness. Refer to my edit a few posts back. Sorry for any confusion I've created here. Thanks to those who stated the contrary.

 

Cold tirepressure advice is all to give the tire a maximal heatproduction wich the cooling down can handle so not any part of tire overheats.

I am able to calclulate it for you, with even saver formula then the official european , I got hold of end 2007, and went running with.

Need tire specifications , maxload or loadindex, kind of tire to determine the referencepressure, here Standard load and XL/extraload/reinforced only used, for Porche with low aspect ratio tires ( hight/width division) mostly XL with referencepressure 42 psi, wich is not the maximum cold pressure written on tire.
and need speedcode, not verry important , because have my own system for that.

Then the real weights on seperate tires, but axles can do, succes with determining that, the most tricky part in it all.
And maximum speed you use, and wont go over for even a minute .

Already written here, carmakers use GAWR's and max technical carspeed for it mostly nowadays.

Then the advice is for ambiënt temp of 65 to 70 degrF, I keep it on 68 degrF.
but I made a document to explain and used it on an RV forum and was roosted by the tirrspecialists there. So this is discussable. Can place it here to if wanted.

 

Wanted.

Thank you.

 

I will give the link to the RV topic. In wich 17 of the 169 posts of me.
https://www.irv2.com/forums/f258/adj...er-558461.html
So I dont hyjack this topic to much.
my posts,
#17
#39
#49
#58
#125 , in wich the long explanation.
#129
#131
#136
#137
#141
#146
#147
#149
#153
#160
#162
#164

#15 of RoadTrip 2084 is yust the other way around about what to fill in a heated garage 68 degrF , for use in extreme cold ambiënt temperature.

Rv' s are completely different vehicles then Porches, so yust translate the high pressures like 80 psi to the 30 to 40 psi used for Porches.

If you want to discuss about it , you can also do it by mail , hotmail.com adres with username jadatis ( combine yourselves, spamm robots cant this way)

 

Humble pie?! You sir are a class act, the typical internet behavior is to fight to the death and eventually invoke a certain WWII era leader before admitting any mistakes

 

Thanks. You are very kind!!!

I've always believed in the energy of the air inside the tires rather than the quantity but somehow in the midst of this discussion I got my head screwed on backwards.

Crap happens and I really do appreciate your understanding. I also appreciate those who took the time to call me up on this. Well done ... Well done!!!

 

Thank you Wayne for your insights, openness, and humility. This can be a tough forum sometimes. (Lots of "testosterone", perhaps.) We appreciate you.

 

BTW, I’m no scientist or expert in the topic, PV=nRT starts to max my knowledge of gasses (and to my daughter - no, you don’t want me helping with high school chemistry homework). But it is an interesting thought to me, targeting hot tire temperatures using the 997.2 TPMS (assuming it’s as accurate as it seems to be) and working backwards. Could we deduce a good starting pressure at various temperatures? Then again, I generally add too much air at gas stations with warm tires, then reduce the pressure in a cold garage a day or so later - not exactly a fast process for experimentation.

One more question: I know it’s generally better to overinflate a tire than underinflate, but then again it’s the inside edge of the rear tires that goes so quickly with these cars. Will lower pressure meaningfully add life to the rears, by letting the weight spread over a greater width of the tire? Does anyone have any anecdotal evidence of lower pressures increasing tire life? I know it’s usually the other way around…