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This post on the TireRack blog http://blog.tirerack.com/blog/docs-advice-on-tires/what-is-the-best-tire-pressure-for-my-new-tires recommends that tires be inflated to the vehicle manufacturers specifications (usually printed on a sticker on the inside of the driver's door) for best safety and performance.
Does this still hold true after replacing the tires with a different, non-stock tire?
The manufacturer's recommended tire pressure is only for the stock or stock-equivalent tires. Depending on the non-stock tire, you could be looking at a big difference.
The recommended tire pressure listed on the sticker in your car door frame is usually optimized for ride comfort and little else. According to this website, you will maximize fuel economy and tire life by airing up the tire to about 90% of the maximum tire pressure, as determined on the wall of the tire.
For example, if the maximum tire pressure is 44 psi, like it is for my car, you can safely fill it up to about 40 psi. Do not fill it up more than this, the 44 psi maximum is the maximum for a reason. This advice would hold for non-stock tires as well. Make sure all these measurements are done while the tires are cold by letting them sit for a while.
If the ride is harsh, you can reduce the psi a little at a time to something more comfortable.
I found a neat test you can perform to see if your tires are properly inflated. Draw some chalk lines on your tires perpendicular to the direction of travel. Drive a short distance in a straight line and look at the chalk. If the inside part of the chalk is worn more, you need to let air out of the tires. If the outside is worn more, add more air. If it is worn evenly, you are at a good pressure for that tire.
While technically a different tire has a different pressure which gets you the best performance on a particular car, there is no way for you to know that pressure, and the recommended pressure is the best estimate you have. In fact, the optimum tire pressure depends on many factors (load and temperature, just to mention a couple). Think of race cars for which the decision on the tire pressure is made on the day of the race, and can even change on a pit-stop. A few pointers:
Increasing tire pressure reduces fuel consumption to some degree. Just remember that going largely out of spec (by more than 0.5 bar) is playing with safety.
Decreasing the tire pressure below the spec increases the risk of aquaplaning. While the opposite is also true to some degree, the recommended pressure is calculated with aquaplaning in mind, so increasing it further won't make your car immune to the phenomenon.
Changing the ratio between tire pressure on front/rear may worsen the handling by making your car oversteer or understeer. Oversteer appears when your rear tires are overinflated compared to front tires (and understeer in the opposite case). Also keep in mind that front/rear ratio is often dependent on the load. For example, my car has recommended pressure of 2.4/2.2 bar front/rear when driving alone, and 2.6/2.8 bar with maximum load.
Personally, I tend to inflate slightly above the recommended pressure to make sure aquaplaning is manageable even if I didn't check the pressure recently and my tires deflated a bit since I last inflated them. I also try to respect the recommended front/rear balance, inflating rear tires more when going on vacation and deflating them when I come back to lone driving. Note that not all cars require increased rear pressure to deal with load: check the manual for your car.
PS. I worked for 3 years in Goodyear R&D center, if you think such things matter.
I had the same question and came across this awesome paper by Toyo Tires (see page 7). This isn't specific to their tires. Basically you use your OEM tire specs to look up the corresponding load front/rear. You then use the same table to find your new tires and reverse lookup the PSI from the same load front-rear.
The example they give:
OEM tire: P225/60R18 93W with door sticker saying 32 psi front / 29 psi rear. The table corresponds this tire/pressure combination to 1354 lbs front / 1272 lbs rear.
New tire 245/40ZR18 97Y to handle loads of 1354 lbs front / 1272 lbs rear, the table says you need 35 psi front and 32 psi rear.
Needed tire-pressure is all about actual load on tire and maximum speed you use and wont go over for even a minute, and a bit about alighnment specific the camber angle above 2 degr wich is sometimes done behind on sportif cars.
I once got hold of the formula the European Tire- and Car- makers use to determine the advice pressure and went running with it, and in time even constructed my own and learned a lot about it.
Now if you have other tires then original and maximum load and or Loadrange is different ( other loadindex and or for instance XL instead of standard load P-tire), then the advice pressures also need to be re-calculated.
If you have aftermarket tires of same sises and Loadrange , though the brand or model is different , then in general you can use the same advice pressures.
For instance you have a 195/65R15 91T tire ( wich I had of European system)for brand A, and you swich to brand B and tire sise and Loadrange stay the same, then maximum load also stays LI 91= 615kg and AT-pressure stays 36psi/2.5 bar. If I would swich to 195/65R15 95T XL(=reinforced/Extraload)= 690 kg maxload AT 42psi, and those values are filled in the formula and it would lead to a bit higher advice pressure for the same load.
American AT pressures are a bit different, Standard load always 35 psi/2.4 bar and XL/Extraload/Reinforced 41 psi/ 2.8 bar.
Replacing the standard tire for a vehicle with a wider one assuming the rim can safely take it, spreads the same load over a bigger footprint. So a tire that is say 10% wider than stock will require less pressure or the ride will be harder and tire wear will be concentrated in the center of the tread. I reduce pressure roughly by the % difference. So if it's say 10% wider, a recommended 32 lbs becomes 29. Works for me.
