Some thoughts:
Originally Posted By: mareakin
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on a 15X7 alloy wheel , the size is 215/60/15 which was upgraded from the OEM of P185/65/15 on a 2007 Corolla.
The manufacturere recommends 30psi for front and rear.
According to Tire Guides, in 2007 only the CE model of the Toyota Corolla came with P185/65R15's, but they came with 15X6" wheels. I'm assuming they are aftermarket wheels if they are 15X7"'s.
But if not, be aware that the 6" rim width is the smallest rim that a P215/60R15 fits on and may be part of the source of the center wear.
Another thought: THEE most important thing about tires is load carrying capacity - and inflation pressure is a major factor when it comes to that! When vehicle engineers size tires and select inflation pressures, it's where they start.
One of the lessons learned from the Ford / Firestone situation a few years ago is that the load carrying capacity of the tire's size / inflation pressure combination has to be larger than previously thought. In other words, there has to be some unused capacity.
The load carrying capacity of a P185/65R15 at 30 psi is 1088#. That means the load carrying capacity of the front tires was 2176. At the published GVW of 3585# and assuming a front / rear weight split of 60 / 40, the front load on the tires is 1971#, so the tires have about a 10% unused capacity - which seems a little small for my taste.
To carry the same load a P185/65R15 does at 30 psi, a P215/60R15 needs to have only 20 psi. (Actually the chart doesn't go that low, so I'm interpolating - plus I do not recommend anyone ever use less than 26 psi in passenger car tires, because that the pressure it is possible to unseat the bead when doing emergency manuvers.)
So using 39 to 35 psi is ...... well ..... way more load carrying capacity than the original tire size / pressure combination. As indicated, this may also contribute to the center wear.
BTW, the tire load tables are predicated on the deflection being the same for all points on the table. One of the fallouts of that is that the footprint size will be the same for all those points. This means that using proportionally more inflation pressure reduces the size of the contact patch.
Plus, more inflation pressure raises the spring rate of the tire. Changing the pressure split would change the handling characteristics. So using a 38 / 35 spilt on a car calling for 30 / 30 is going to result in a car with less understeer. Plus using a higher pressure is going to increase what we tire engineers call "aligning torque". It's the force that pulls the tire back into position when the steering wheel is turned. But the inverse of that is that less slip angle develops for a given steering input - and that results in a more precise steering feel to the car - call it "crispness".
So overall, the car is going to feel more balanced and precise.
However, this has a down side. Understeering cars communicate their reaching the limit of adhesion by giving you plenty of warning. Increase the inflation pressure, and the limit of adhesion is approached more abruptly. Make the car have less understeering and the approach to the limit of adhesion is less noticeable. Overall, vehicle engineers consider this less safe, as the warnings that the limit of adhesion is being approached become less obvious.
So that's some of the science behind the tire pressure thing.
A word of caution: Vehicle engineers spend coutless hours driving their cars on their test tracks in order to find the little handling quirks. So be cautious about using an inflation pressure different than what is specified.
