Cold Tires in Warm Weather Pressure Question

Okay, I know there have been a number of threads on this before, but none that I can find answer the following question. At what pressure should I inflate my tires if (1)the ambient temperature is 90 degrees, (2)the car has NOT been driven ["cold" tires so to speak], and (3) recommended cold tire pressure at 68 degrees is to be 30/31? Most all the threads speak to what happens when my tires warm up due to driving, but none speak to filling them when the car has not been driven but the temperature is nonetheless high (as has been the case this whole damn summer!!!). Put another way, at night where I am the temp only goes down to about 80 degrees and quickly gets to 90 and beyond. So, do I fill them to 30/31 even when cold but the ambient temp is 90, or do I take the inflation levels down by 2 degrees (assuming a ratio of one degree for each ten degrees of ambient temperature over 68)?

I would not reduce the cold pressure when it is hot out. When the temperature outside drops, your tires will be below recommended. When you are driving, the tires heat up, how much depends on the speed you are going, how many turns you are making, how much braking you are doing, etc. I have to presume the Porsche engineers know what they are doing when they give out their recommendations. If your recommended pressures are 30/31 F/R, leave it there. You may have to add some air when it is cold out and possibly release some when it's hot, but I wouldn't go less then the 30/31.

Here's the problem. The placard on my car specifically calls for 30/31 at 68 degrees. Yes, this certainly reflects their engineering expertise, but it also provides no guidance if the ambient temperature will never (during my summers) be less than 80 degrees. Of course when the weather cools I can and will adjust the pressure, but for now - while the weather remains warm all the time, I am still looking for the correct pressure. BTW, this question goes the other way. When the weather gets cold (well below 68 degrees) should I raise the pressure above 30/31 or merely re-inflate back to 30/31, given the pressure in the tire will have dropped?

I like the way you're thinking about this, but I would not bother with lowering initial pressures in order to adjust for subsequent higher ambient temps in that case. If the low temperature only gets down to about 80 degrees, 90 degrees or so is not much of a gain anyway. Tires are designed to handle the natural psi gain better than they can handle under inflation. I would be more concerned if the tires lost pressure overnight due to a cold snap. In that case, I'd bring the pressures back up to where I usually want to start them. You must have one of the new cars. We're not used to seeing recommended pressures of 30/31. Well, two posts came while I was typing. The 68-degree standard applies to our "old" cars and tires as well, but I'm not going to do the math every time I'm about to drive the car (I do check tire pressures though).

So where do I put the pressure to start, assuming a low of only 80 degrees? Do I move it up to 31/32?

I think it's more like a 0.5 PSI increase in cold initial air pressure going from 68 to 80 degrees. It's described by the equation P1*V1/T1 = P2*V2/T2 where P=atm, V=liter, T=Kelvin It's not possible to calculate the change of volume of the tire due to change in air pressure (at least I can't), so you can leave that out.

I don't think that it matters. Intuitively, the increase in pressure OVER THE INITIAL COLD PRESSURE by driving is probably rather constant. So, setting cold pressures at 30/31 in cold weather will result in the same increase in pressure during driving as setting them at 30/31 in hot weather with the same driving. The problem really would be setting the cold pressure at 30/31 in hot weather and then not monitoring the pressure as it gets colder out - the pressure will naturally fall and needs to be readjusted upward. The cold tire pressure really doesn't mean cold - it means "before use". I would check pressure in my car everytime the ambient pressure changes by more than 5 degrees. The idea is to keep it set at the 30/31 year round before use. For me, here in Nova Scotia, cold pressure set at 30/31 in summer could fall to 25/26 in winter and in between in spring and autumn. Not to worry, but check it periodically. But keep in mind that "cold" pressures set at 30/31 in winter will be a lot higher 'cold" in summer - maybe 36/38.

Don't over complicate it. I'm pretty sure the manufacturers never intended average users like us to start calculating how to adjust our tire pressure every time the season changes or there is a several degree change in temperature. The intent and instructions are for us to check the pressure regularly, I do mine weekly, and set it to the recommended pressure. Nothing tricky there. Outside temperature can change the reading and we should just always adjust to the recommended pressure, making sure to only do so when the tire is cold/not yet been driven on. Done.

^ What he said.

I think the theory is that the tires are going to heat up by about the same delta temp, so starting out with the same "cold" pressure will result in the same "hot" pressure, with the understanding the "hot" is cooler on a low temp day.

Yep, same here. Set it to recommended. Or what you've found gives the best balance. The only time I deviate from this is winter when I'm checking in the garage at 50° and it's 20° out for instance. Then I set 2-3 psi high. The rule of thumb I've always seen for the pressure change is approx 1 psi for each 10°F.

Easy use this stuff: Seasonal Tire Air [1cas] - $29.95 : KaleCoAuto, Hard to find automotive items!

For years now I've set my 02 Boxster's tires to the 29/36psi front/rear inflation pressure just making sure car has sat a while -- overnight is best -- to make sure tires are 'cold', as cold as they can get. If the outside air temperature is 32F I set the 'cold' tire pressures to 29/36. If the outside air temperature is 90F I set the 'cold' tire pressure to 29/36. The only thing I've run into arises from wild swings in temperatures. What can happen is hot weather is the norm -- for weeks on end -- and tire pressures are while set 'cold' cold in this case can be 60F, 70F, or even higher. Then a cold spell comes in. Now tires are under-inflated. (Roughly a tire loses 1psi for every 10F drop in air temperature.) Or it is cold cold cold and tires are inflated for this cold weather. Then the weather gets warm. In this case the tires will be over-inflated. So when the weather is changing you have to stay on top of things but it is not hard. Or I would leave the mid-west in winter with snow/ice on the roads and drive west for 2K miles and in some areas encounter rather warm air temperatures. Or leave the west coast with mild weather in effect and then overnight in Flagstaff air temperature drops to 19F overnight while car out side of motel room. I've found that with my Boxster if the fronts are over-inflated a few psi the car develops a vibration that feel sort of like a tire balance problem. These cars are quite sensitive to tire pressures so buy a good easy to use tire pressure gage and use it to keep the cold tire pressures right. The hot pressures will take care of themselves. Sincerely, Rockster.

This is the best answer that I have heard so far. Cold means "unused". Hot means "in use". The temperature is irrelevant.

That is really funny! Seasonal Tire Air at $29.95. Is anyone selling cans of air from Weissach or Zuffenhausen?

It's been a long time since high school chemistry but I believe the Ideal Gas Law can offer some guidance. The Ideal Gas Law states PV=nRT where: * P is pressure in pascals * V is the Volume in cubic meters * n is the number of moles * R is the gas constant * T is the temperature in kelvin Re-arranging the equation to solve for pressure gives P = (nR/V)T. In this case, nR/V is constant so the equation can be simplified to P = CT. I'll use these values to solve for C. 30PSI = 206,842 Pascals 68F = 293.15 Kelvin 206,842 = C * 293.15 C = 206,842/293.15 = 705.584 Now that I know C I can plug that value into the equation P = CT to figure out what the pressure will be if the air temperature is 90F and also assume that the air temperature inside the tires is equal to the air temperature outside the tires . 90F = 305.372222 Kelvin P = 705.584 * 305.372 = 215,466 = 31.25 PSI Assuming I'm remembering how to correctly apply the Ideal Gas Law, if the temperature is 90F and you want to inflate your tires to the equivalent pressure of 30PSI @ 68F you should add 1.25PSI to the recommended pressure. My personal thought is that for day to day driving, plus or minus 1.25 PSI isn't going to make much difference. You may see better or worse gas mileage and slight differences in tire wear, I wouldn't worry about it. If this is for the track, slight differences in pressure could be more critical and will defer to those with more experience than myself.

Caymanites: I've read all the posts and I vote for LES!!! I've never heard of a cold pressure listed "at 68 degrees". I think they mean "At ambient temp", which would be cold tires , car parked outside. If you have a heated garage, (I heat mine to about 45 degrees), you need to set them higher than recommended in the garage if ambient in the garage is much higher than outside (in winter). 10 degrees = 1 psi When it's below zero here, I don't want to have a tire problem. In summer, I have A/C, but only use it to keep the upstairs habitable. The space where the cars are gets a little help from that on a hot day...maybe to 80 degrees when it's 90 or above. Not much temp difference. If there was, I'd set the air a little lower than normal because the tires will heat up more than normal just parked outside (ambient). I also use 1 psi for every 10 degrees temp difference. That may be a little high. Maybe 1 psi for 15 degrees is closer. You get a feel for it after a while. When I track my car, I don't care about cold pressure. I set the tires to recommended, go out for a couple laps...enough to really heat up the tires , and pit to check them with a pocket pistol grip digital gage I have. I shoot for 38 psi all around. Invariably, I'm letting air out to get it. It takes a full session to get everything to 38. Then I park the car and leave it along until next session. By then, it's pretty close. I'll check again if I stop and do a little fine tuning of a pound or two to get it feeling just right. When it's close, it's not difficult to feel 1 psi. change in either the fronts or the rears. As the day wears on, the track gets hotter. My tires , NT-01s, don't work well if they get inflated to over 40 psi, so I have to keep checking and letting air out in the afternoon. By last session, my car is so dialed in. Then it's off to the motel. I do not change my air to drive to the motel. Sunday morning, it's generally colder and so is the track. My tires are pitifully underinflated if you look at cold pressure. I'll add 3 psi or so to each tire to start and then begin the process of setting hot pressure over again but using my tweaked pressure instead of 38F/38R. At the end of the school, I pack up the car and, last thing is to set cold pressures to recommended for the trip home. Cold pressures just don't work at the track. Great for street where the only extreme driving is high speed driving and not warp speed turns and braking continuously. Driving on track tires going home from the track, I just want to get it home and get them changed. Used to take the back ways home, but now I take the biggest roads I can find. The brake noise from stopping and starting track brakes makes me nuts. When I get home, I put the car on my lift :banana: and take all the wheels off, then do all the brakes then put all the street wheels back on. Takes about an hour if I pour over the work. Includes putting things away including the tires . I've gone on two sport touring motorcycle weekends in sports cars, first my old M3 and second my CS. All day, no A/C, windows open and no radio, just ripping on miles of abandoned back roads. I had R tires on the M3 and the bikes couldn't keep up. In the CS, I had street tires and the bikes couldn't keep up. :burnout: Even those wild rides didn't heat the tires like a track day does. I just left them at recommended pressure. They worked fine. :cheers:

Take a look at these references in the 2010 owners manual: Pg. 151 The Tire Pressure Monitoring offers the following functions: – Display of the actual tire pressure while the vehicle is in motion. – Display of the deviation from the required pressure (refilling pressure). – Display of currently set tire size and type. – Tire pressure warnings in two stages. Pg. 152 & 153 Pressure info in tire pressure menu In accordance with physical principles, the air pressure changes as the temperature changes. The tire pressure increases or decreases by around 1.5 psi (0.1 bar) for every 18 °F (10 °C) change in temperature. The Tire Pressure Monitoring takes this relationship between tire pressure and temperature into account. f Please see the chapter “PRESSURE INCREASE AS THE RESULT OF TEMPERATURE INCREASE” on Page 160. The tire pressure to be corrected (refill pressure) is indicated on the displayed wheel. Example: If “ –1.5 psi (–0.1 bar)” is displayed, 1.5 psi (0.1 bar) must be added to this tire. Note The tire pressure menu can only be called up when the vehicle is stationary. Calling up the “Info pressure” display f Push operating lever forward in order to switch on the selection field. f Select “TPM” with the operating lever. f Push operating lever forward. The display changes to the tire pressure menu. f Select “Info pressure” with the operating lever. f Push operating lever forward. Note After the ignition is switched on, it can take up to approx. 1 minute before all tire pressures are displayed. Dashes (“-.-”) appear instead of the tire pressures. Hope this helps:cheers:

Andover: Sounds like a bunch of engineers with time on their hands. Who asked for this innovation??? Talk about solving a problem that no one has! :crazy: All I need to know is when a tire is going down from a puncture so I can repair it before it destroys itself or the car.

Actually, my experience has been the opposite. I've found my Cayman S to be the least sensitive to tire pressure changes of any I've owned, whether on the street or on the track (up to when the tires get greasy). In contrast, my 911 would light up my butt meter if a tire was 1-2 lbs off. When the fronts are over-inflated by several psi my car may get a bit darty or stiff, but there's no vibration. For the street, I run 32/36 cold regardless of outside ambient temp. However, I've got a thermometer in my garage and will adjust my "cold" pressures based on +/- 1psi/10 deg expected difference between garage and outside temps.