What's the Right Tire Pressure?

[Jack Tripper]

I'm rolling with 19" Slipstreams and read the proper pressure is 45 psi. But in my CID display it says my tires are 41 psi when I start driving, and around 30 minutes later the TPMS says they're now at 45 psi instead.

Are my tires under inflated or just right? I just got my car in December.

 

[Terry_B58]

Tyre pressures should be checked when cold, as they warm up the pressure will increase.

There should be a sticker on your door pillar stating what the pressure should be set at (cold).

 

[NickFie]

Tyre pressures should be checked when cold, as they warm up the pressure will increase.

There should be a sticker on your door pillar stating what the pressure should be set at (cold).

You can set cold tire pressure anywhere between the number on driver’s door pillar - bottom of range - and Max PSI number on tire’s sidewall.

Lower than minimum and you risk tire damage and blow-out during sustained high speed driving, particularly in hot weather. Higher than max and you risk tire exploding off the rim.

Lower pressure in range gives smoother ride, somewhat better traction in mud and sand. Low enough and tread wears faster at edges.

Higher pressure gives better range, sometimes better handling. High enough and tread wears faster in center and traction is worse.

I’ve seen posts saying 50 psi cold works well. Currently aiming at 48 psi cold. Tires are at 45 psi on cold mornings.

 

[jerry33]

Always check tire pressures cold first thing in the morning before driving (or less than a mile if you don't have a way to do it at home. The vehicle placard pressure is a starting point but needs to be adjusted (usually upwards for on road cars) depending upon conditions. In the winter, if your car is garaged, you need to make an adjustment for the lower outside temperature. In summer you need to make an adjustment to compensate for the higher temperatures.

All tire pressure recommendations are based on a few assumptions, which may or may not equate with your driving conditions:

1. Temperature is 18C (65F). At 38C (100F) add 4 psi. For vehicle placard pressure of 310 kpa (45 psi)

2. Speed is faster than 105 km/h (65 mph) add 3 psi (Tesla most likely already does this).

3. Load is what the engineers thought the car would be at most of the time--this may be lower than the GVW. (Add a bit when traveling with the family on vacation.)

Note that this is very different from tire pressures for track use, but then tires used for track aren't expected to last very long.

> How much does the pressure increase just from driving?

The correct answer is: it depends.

What it depends on is the starting inflation pressure, tire size, and
vehicle weight.

The actual process is that the tire flexes and creates heat. This raises
the tire pressure, reducing heat creation. This cycle of flexing--pressure
increase--reduced flexing continues until the amount of heat generated is
equal to the amount of heat lost. At this point (called thermal
equilibrium) the tires will not gain any additional pressure.

What you are shooting for is no more than a 10% to 15% pressure increase
from driving. There is also a pressure increase from a rise in ambient
temperature (hot sun, black tires, this can be 10% or so).

If your tires are 25 to 50% low (unfortunately very common), you can
easily get way over 25% pressure increase (you will be able to fry eggs on
the tires, worse the compounds will start to deteriorate).

It's normal to put additional pressure in the tires in hot climates.
Getting this right can, in some cases, double the life of your tires.

 

[jerry33]

Higher pressure gives better range, sometimes better handling. High enough and tread wears faster in center and traction is worse..

Centre wear is very uncommon on most of today's tires. It was a big concern in the days of bias-ply tires. What it depends on is the crown radius of the tires. Many tires today have a flat, or almost flat) crown radius and don't exhibit centre wear.

Traction on dry pavement will decrease, but will be better on wet pavement due to the reduction of the possibility of hydroplaning. On ice, higher pressures will wipe away the film of water that covers the ice more effectively, actually increasing the traction (It's the film of water that makes the ice slippery). This is why track pressures are lower than street pressures.

 

[FlyingCookie]

I’ve found that 53PSI after ~30 minutes on the interstate works well. Gives me good range and pretty even tire wear, but this means my cold pressure can vary between 45PSI and 51PSI, depending on how cold it is outside.

As I drive a lot, and may leave an area that’s 80F and arrive somewhere 12 hours later at an area that is 20F, I carry an air compressor with me and adjust tire pressure as needed at Supercharger stops.

 

[rory breaker]

How about 21s w pilot ss’ - I’ve seen diff opinions on this forum, what’s the right answer?

 

[jerry33]

How about 21s w pilot ss’ - I’ve seen diff opinions on this forum, what’s the right answer?

That's because there is no one right answer. Use the general principles above and then watch what the tires do.

 

[Graffi]

I try to keep my pressure at about 48 psi "cold" when the weather is moderate. This way it will increase to about 50 to 53 psi when on trips. Seems to work well for me. Just rotated tires at 20,000 miles and still had 5/32 left all around and across the tires, original tires. No need for snow tires in San Diego, and have never had the car in snow/ice. If I did, then I would maintain two sets of tires. Eventually we will wear out the tires so why not go ahead and invest in your second set for cold winter driving (but not in San Diego, lol). It seems that every few weeks we will have a few days in the mid 80's F.

 

[NickFie]

Centre wear is very uncommon on most of today's tires. It was a big concern in the days of bias-ply tires. What it depends on is the crown radius of the tires. Many tires today have a flat, or almost flat) crown radius and don't exhibit centre wear.

A few years ago, we increased tire pressure about 5 pounds above recommended pressure on my brother-in-law's Avalon before 800-mile trip to compensate for the weight of 4 adults plus over-stuffed trunk and passenger compartment. At my suggestion, he maintained that pressure after the journey. He later reported that the tires wore more quickly than expected, especially at the center of the tread.

Sometimes you can still overinflate.

A quick check if your car doesn't have TPS, or it doesn't report individual tire pressures. When you stop after an hour or so at highway speed, feel each tire with the back of your hand. If pressure is too low, the tire will be quite warm. If one tire is warmer than the others, its pressure is too low. I tend to do this at nearly every stop in non-TPS vehicles.

 

[JonG]

I can’t believe the advice written here by some.

Tyre pressure increases in use due to flexing of the tyre wall increasing the temperature relevative to the ambient. The advice is to set the tyre pressure when at the ambient temperature (ie when cold and not warmed up through use) knowing they will change.
A lot of engineering is done to calculate what the correct tyre pressure is and this is what’s stuck on the b pillar of the car, for 19” ms wheels it’s usually 45psi when cold/ambient but after operation will read higher than this.

This is therefore the correct pressure to set your tyres.

Because the pressure changes with temperature, when the ambient changes significantly, so will the pressure in the tyres. In cold weather a 10 deg C/20 F change equates to about 2 psi. On a cold day you need to add air to return your tyres to the correct pressure. On a hot day, remove some. In practice we accept that it’s rare the pressure is exactly right but setting your pressures monthly, which is good motoring practice, is ok unless something freakish happens to the weather.

It’s also thought better to err on the side that slight over inflation is better than under inflation, so an extra 1 or 2 psi if you’re setting the pressures at the hotter part of the day is accommodating the fact the pressure will drop and therefore the pressure.

Over inflating reduces flex in the sidewall which can reduce grip, increase bounce, cause increase centre wear etc.
Under inflating can increase drag, and increase wear at the edges.

 

[NickFie]

I can’t believe the advice written here by some.
A lot of engineering is done to calculate what the correct tyre pressure is and this is what’s stuck on the b pillar of the car, for 19” ms wheels it’s usually 45psi.

This is therefore the correct pressure to set your tyres.

Engineering involves compromise within technical limitations. Tire pressure is an example. The engineers find a pressure that provides the desired mix of comfort (low pressure), traction, handling and fuel economy (high pressure). If you prefer a different compromise point, then a different pressure may do better.

Two examples:

1. My wife drives the Tesla - it was her birthday gift. I'm the long-haul driver, also tech wrangler. My daily driver is a Hyundai Sonata 2.0 Turbo. The driver's door placard says 35 psi. 42 psi works best for me, still below sidewall 50 psi limit. Better fuel economy, better handling and even tire wear. The firmer ride reminds me of driving dad's Triumph years ago.

2. If you find a European site for Michelin tires, you'll likely see a set of tire pressure adjustments for various highway speeds. Even the vendor acknowledges there is no single correct number.

 

[MelaniainLA]

So in 90 degree Los Angeles summer where X says 45psi but readings are consistently 41psi “cold” in AM, what pressure should I be aiming to achieve??

 

[murphyS90D]

So in 90 degree Los Angeles summer where X says 45psi but readings are consistently 41psi “cold” in AM, what pressure should I be aiming to achieve??

45 psi "cold". All tire pressure specifications are when the tire is cold.

 

[MelaniainLA]

45 psi "cold". All tire pressure specifications are when the tire is cold.

Thanks!!! How long after driving slowly for 6 miles would you count it “cold” if garages outside of heat?

 

[daktari]

Thanks!!! How long after driving slowly for 6 miles would you count it “cold” if garages outside of heat?

I don't quite understand you. But they will heat up pretty fast, 6 miles is too long a drive to consider them cold/ambient.

Change your IC to show tire pressures on start-up and watch the temp increase after just a few minutes.

Btw, I reduced pressure on mine (19") to 41 psi to get a more compliant ride. Works great, comfort is better. Speeds at our highways are low here so I feel safe.

 

[MelaniainLA]

I don't quite understand you. But they will heat up pretty fast, 6 miles is too long a drive to consider them cold/ambient.

Change your IC to show tire pressures on start-up and watch the temp increase after just a few minutes.

Btw, I reduced pressure on mine (19") to 41 psi to get a more compliant ride. Works great, comfort is better. Speeds at our highways are low here so I feel safe.

Thanks! So what I meant was, after I have driven for 6 miles, how long should I let the car sit undriven before I can reliably check and adjust the tire pressure without it falsely being elevated due to recent use?

 

[jerry33]

Thanks!!! How long after driving slowly for 6 miles would you count it “cold” if garages outside of heat?

Cold is no more than one mile driven slowly. Once the tires are no longer cold, they need to sit for eight hours. Tires lose heat very slowly.

 

[romeoecho]

My door jam sticker says 50 psi cold.. so, that's what I try to maintain. I overfill to '51' at digital pumps (such as WaWa) as they tend to not be accurate and always over (at least here in FL).

 

[Oldschool496]

This guy is hitting all the right notes. The engineering and testing is vast. 45 is printed and should be checked as needed. They over-inflate on their own during trips. If fully loaded vehicle you should never over-inflate, it just gets way out of control.

45 is the best all around number (cold) for energy usage whether Electric or Gas, Tire wear, Heat, Traction. Might at times be harsh ride, but its the number. The highway patrol in Florida and California uses that number, they know quite a bit about tires and the best characteristics for best all around results.

Now, not all tires can take 45 going to 50 in heat, thats an entirely different discussion. Use what is printed on the pillar along with the right tire.

I can’t believe the advice written here by some.

Tyre pressure increases in use due to flexing of the tyre wall increasing the temperature relevative to the ambient. The advice is to set the tyre pressure when at the ambient temperature (ie when cold and not warmed up through use) knowing they will change.
A lot of engineering is done to calculate what the correct tyre pressure is and this is what’s stuck on the b pillar of the car, for 19” ms wheels it’s usually 45psi when cold/ambient but after operation will read higher than this.

This is therefore the correct pressure to set your tyres.

Because the pressure changes with temperature, when the ambient changes significantly, so will the pressure in the tyres. In cold weather a 10 deg C/20 F change equates to about 2 psi. On a cold day you need to add air to return your tyres to the correct pressure. On a hot day, remove some. In practice we accept that it’s rare the pressure is exactly right but setting your pressures monthly, which is good motoring practice, is ok unless something freakish happens to the weather.

It’s also thought better to err on the side that slight over inflation is better than under inflation, so an extra 1 or 2 psi if you’re setting the pressures at the hotter part of the day is accommodating the fact the pressure will drop and therefore the pressure.

Over inflating reduces flex in the sidewall which can reduce grip, increase bounce, cause increase centre wear etc.
Under inflating can increase drag, and increase wear at the edges.