# Nailing down "cold inflation pressure"

Everyone’s read the phrase, “Cold inflation pressure: 35 [or whatever] PSI”. But I’ve never come across a concrete value for what “cold” means, temperature-wise. Many times I’ve read that it’s the temperature your tires are at in the AM when you go to work. That could be anywhere from 80 degrees F to [-30F] depending on the time of year & the location.

I’ve also head that tire pressure increases 1 PSI for every 10 degree increase in temperature. Is this true? Maybe Capri racer is the resident expert on this subject, but I’m throwing this out for him & everyone else.

I want to start using an infrared thermometer to properly adjust tire inflation on my customer’s cars. I work in a slow tire shop & I have a lot of time to spend per car. Any input would be appreciated. KS

Find something else to do…The tire pressure / air temperature equation exists but exact tire pressure is just not that critical. There is a considerable amount of slack built into tire pressure recommendations…Nit-picking tire pressure is not necessary…

It’s the first thing in the morning pressure. So it applies if you’re in Fairbanks in the winter or Dallas in the summer. Only if you drive from a winter climate to a summer climate during the day (or vice versa) might you need to think about it.

One of the reasons why you should check your tire pressure is because it will change as seasons change. Cold pressure is when the car has not been driven for a while and the tires come to about the same temperature as the environment. It is often easiest to test the tire pressure in the morning for that reason.

“There is a considerable amount of slack built into tire pressure recommendations…”

True enough, but not in some tire pressure monitoring systems.

“The tire pressure / air temperature equation exists…”

OK, so what are the values? For the sake of knowledge, if not practicality.

Now if you’ll excuse me, I have to do a rear wheel alignment on a hook & ladder…

That’s not a bad question. I agree with “jt”. The cold tire pressure, we would assume would be the pressure of the tire at the ambient temperature regardless of what that temperature is in it’s environment prior to operating. Anything after that when the tire is subjected to the heat build up while in motion would not be cold.

Thanks for your comment dagosa. I believe you & jt are right. Just for jollies I’m gonna test how much the pressure changes on a hot day. It was 101F Friday here in SE PA!

Cold pressure, as you may see from the various answers is measured before you drive for the first time on any day. The next thing to be concerned with is tire pressure gauge accuracy. Buy a quality gauge to keep only as a standard to compare to the daily use gauges that are subject to various abuses including moisture, dirt, being dropped, frequent usage, and having other tools dropped on them.

I suspect the new tire pressure monitoring systems are set to trigger a warning at some minimum tire pressure a few PSI below the recommended pressure. They remain silent if the tires were over-inflated, either intentionally or by accident. Many people adjust tire air pressure according to the type of driving they expect to be doing and the weight of the loads they intend to carry…These air pressure adjustments are not fine enough to require infrared equipment…But hey, today’s automotive service customers are easy to impress…They were lined up to have nitrogen pumped into their tires…You don’t see that much anymore…

Cold tire pressure at 7AM is different than cold pressure at 3PM when a vehicle may be in the shop. I add 3 PSI during the heat of the day when filling tires and 5 PSI if the wheels/tires are hot from being driven within the last hour. If the vehicle is left over night the pressure checks out to be very close to the tire pressure label.

Toyota has a chart to adjust from outside temperature to shop temperature. http://www.toyotapart.com/TIRE_INFLATION_PRESSURE_COMPENSATION_AND_ADJUSTMENT_T-SB-0345-08.pdf

"Many people adjust tire air pressure according to the type of driving they expect to be doing and the weight of the loads they intend to carry."
Where I live few people adjust their tire pressure for any reason. Most people complain that the tire pressure warning light has been on for three months. The most common complaint with a service visit is that the tire pressure warning light is on. So much for government mandated saftey systems.

Put it this way, the pavement temperature on a sunny day in Maine can reach 140 F or hotter. Don’t adjust the pressure if you just parked the car in a sunny place 15 minutes ago and the time is 2:00 PM. Glad you have an IR thermometer. Don’t believe anything like “the pressure increases so many PSI per 10 degrees increase in temperature” unless you read it yourself in a text book. So many rumors, so little time.

Tire makers know about pressure increases, so after a lot of years they recommend how they should be checked. We all don’t have to have every answer in the universe.

First, the word “cold” means “at the ambient temperature” (outside temperature) and not heated due to operation. Ergo, any tire that has been operated has built up a little heat (or a lot), so the pressure reading is elevated - and you can’t tell how much unless you take the tire’s temperature.

Second, as the outside temperature increases, so does the pressure - about 1 psi for every 10°F. It is common for folks to recommend that tire pressures be taken in the morning so the tire is “cold” (un-operated). The fact that the outside temperature rises during the day complicates matters, but we are usually talking about a 20°F rise, so that’s only 2 psi! Not enough to worry about.

What is of concern is if the tire’s pressure was adjusted when the outside temperature was 80°F, and the weather changes to 30°F - a 50°F drop (5 psi)

What is also not usually known is that new tires will grow. It takes about 24 hours for most of the growth to take place - and that affects the size of the air chamber - about 2 psi worth. So if you are in the retail tire trade, it isn’t a bad idea to set the presure 3 to 5 psi high to account for all those factors. However, used tires - particularly tires already inflated - have already experienced this growth, so no adjustment for growth is needed.

And just to reiterate a point, the vehicle tire placard will list the original tire size and the proper pressure for that size. If the tire in question is the same size as what is listed on the vehicle tire placard, then the pressure listed there is appropriate. However, if a different tire size is used, then the pressure has to be recalculated. That requires the use of tire load tables, which are difficult to find on the internet because they are subject to copyright laws. However, I have them and would be happy to do the calculation.

Buy a quality gauge to keep only as a standard to compare to the daily use gauges that are subject to various abuses including moisture, dirt, being dropped, frequent usage, and having other tools dropped on them.

Seriously? Frequent usage constitutes abuse?

I won’t even start in on the other stuff.

"It was 101F Friday here in SE PA! "

It was 105F here last Friday; 123F wet bulb.

Equating the tire’s pressure to the ambient temperature in the morning really makes a lot of sense if you think about it. A tire’s pressure from use will change roughly the same whether it started the morning at 101F in Arizona or -30F in North Dakota, So starting both out at the same pressure at ambient makes sense. I said “roughly” because the variable of the change in ambient temperature as the day progresses is always a variable.

We always harp on monitoring tire pressures, but the tires are well able to safely operate within a range that can be expected throughout the course of a normal day. They can even accomodate pressure adjustments to accomodate different needs or preferences, such as lowering pressure to drive on soft sand or even running them a bit high to accomodate the extra weight when you load the vehicle to its hilt. It’s when the pressure is allowed to drop too low that problems develop.

"jtsanders 11:46AM Report
"It was 101F Friday here in SE PA! "

It was 105F here last Friday; 123F wet bulb. "

I hope you have a quality thermometer to keep only as a standard.

STP, standard temperature and pressure, back in my high school science class, was 72°F at 29.92 hg. It seems now though that 20°C or 68°F is usually considered ambient temperature. At least it is in the electric power industry.

Edit, I just googled STP, wow there are a lot of standards but 0°C or 32°F are very common. But I also see a lot of 20°C or 68°F as well as a number of others including 59°F, 60°F, 70°F and 25°C. Even standard pressure varies a little.

There exist a number of “standard conditions” depending on what industry one’s in.
http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure#International_Standard_Atmosphere

Setting the temperature issue aside, I always wondered what was meant by Max pressure. Should you inflate to that pressure, or should you leave room for heat expansion?

Never inflate a tire to max pressure. Always inflate it to the pressure on the doorjam, or get the pressure info from your owner’s manual.