What is the optimum tire pressure?

I agree with ceric IF the psi starting point is at 100% contact with a radial tire. The simple equation is totally misleading since the contact area is not a linear function. Ed
 
The door sticker recommendation is the best all-around pressure for safety, steering response and tire life. If you deviate much from this recommendation you are compromising one for another. Ed
Ed has it right. Most of the replies did not address the OP question or "optimum"

"Optimum" is what the door jam says. It's not the 'optimum' for wear, not the 'optimum' for mileage, not the 'optimum' for handling, not the 'optimum' for best ride, Bbut it is the optimum to get the best combination and trade-off all three.
 
There's cold tire pressure and then there is cold tire pressure. A little story:

Toward the end of washing and interior detailing my Sienna last year on a somewhat hot and very sunny afternoon I noticed the tires facing south toward the sun were quite warm. The vehicle had been sitting in the driveway for maybe 2-3 hours. The north-facing tires in the shade of the vehicle were cool as one would expect since the only driving in the previous 24 hours was backing it out of the garage.

The warm tires read 2 lbs. higher just from being heated up by the sun. The cool tires were at a spot on 35 per the door label. I check / adjust tire pressure pretty frequently, probably not long before this reading. Did I forget to do two tires last time? That is not the most plausible explanation given those south-facing tires felt like the car had been driven.

Long story short, cold is not necessarily cold and 2 lbs. one way or the other is not going to make much difference if the sun alone can cause it.
 
Ed has it right. Most of the replies did not address the OP question or "optimum"

"Optimum" is what the door jam says. It's not the 'optimum' for wear, not the 'optimum' for mileage, not the 'optimum' for handling, not the 'optimum' for best ride, Bbut it is the optimum to get the best combination and trade-off all three.

Somewhat true. But ONLY for the OEM tires. Other tires will have different tread compounds and especially sidewall stiffness. The plackard may be a good starting point, but is in no way applicable to another tire.

A surprising example: my Miata's summer tires are Michelin Pilot Super Sports, a superior sports car tire. If run at the plackard pressure, an emergency lane change results in a crazy fishtail and I almost lost the car. Adding 3 psi tames that and the car is safer. Sidewalls are suspension springs in 3 dimensions.
 
Stumbled onto this thread and decided to check the tire pressure on my newly bought CX-5. On a cold morning, after the car's been sitting all night.. All 4 tires measured ~55 psi!!

Double checked on my air compressor readout and it measured the same. My mistake for trusting the dealership would properly check the tire pressure before delivering the car to me. Haven't really driven it much yet, and it's a good thing I haven't!
I am 5 years a member on this site now, and I can't believe that this is still a common issue with Mazda's.
It continues to be a problem, and Mazda dealers just can't seem to get this right. Sheesh.
 
I am 5 years a member on this site now, and I can't believe that this is still a common issue with Mazda's.
It continues to be a problem, and Mazda dealers just can't seem to get this right. Sheesh.

We drove around for about 250 miles before checking the tire pressure too, smh. Just glad they didn't explode on us going over anything sharp on the road. Really glad it wasn't summer time yet.. who knows what the 90+ degree weather can do to the air pressure in the tires when it's hot enough to fry an egg on asphalt outside.
 
...who knows what the 90+ degree weather can do to the air pressure in the tires when it's hot enough to fry an egg on asphalt outside.
Probably another 3 lbs. or so. Depends on the ambient temp when the cold pressure was set.

Hunter Thompson and Dr. Gonzo pumped the tires to 90 lbs. in their rented Cadillac for alcohol and drug enhanced 4-wheel drifts around Las Vegas. 75 lbs. didn't quite cut it. No blowouts. That was pre-radial tires of course. ;)
 
There's may be about 1 ounce of air in your tire when inflated to spec, less than 1/10 of a pound.
 
The Autopian just put out an article regarding tire pressure and the author advocates for sticking with the pressure recommended by the car manufacturer.
Why Automakers Sometimes Overpressurize Tires And How That Can Affect Your Handling: Ask An Engineer

It should be noted that the article is written by Huibert Mees, an engineer with a rather impressive resume.
His rationale, though, is because its safe. I think that's a premise most here already understand. And the rest of his rationale is because of the control limits associated with the TPMS system.

I run my winter tires with no TPMS sensors. This is my first car to have that capability. It is a step forward, but not something I consider critical. If they are a little more sensitive to pressure drop because I'm running a few psi lower than the spec, or less sensitive because I'm running higher, its still much better than no sensors at all.
 
So lots of interesting thoughts on tire pressure but not one person accounts for the GVWR? To first and effectively decide to change tire pressure from the manufactures recommend tire pressure you must know you vehicle GVWR. then add approximation of added weight to calculate how much you need to increase the psi.
The ambient temperature has great effect on tire pressure and is also part of the allowance that the recommended psi be carefully maintained.

Playing around outside the recommend tire pressure may seem to be ok but they are there for a reason. The biggest reason is how the tire pressure will respond to the braking and handling of the vehicle under adverse driving condition and emergency situation.
 
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