The effect of overinflating a tire is to cause the inside to wear faster than the outside.

What I want to know is if overinflation increases the actual diameter of the tire beyond its normal size, and that is the reason why the middle wears faster.

OR

Is it due to the tire becoming more inflexible with the result being that the middle bears the brunt of the weight?

 

It slightly increases the overall rolling diameter of your tire, with the inside of the tire bearing much more of the weight than the outside. I'm not sure what you mean with the second option there.

This should probably go in the wheel and tire forum though.

 

Yes, like you said, with too much pressure the center bulges outward as it becomes rounder than normal. It's not as big of an effect with modern radial tires but that's still the dominant wear pattern.

However, with low profile tires it's actually possible to see heavy center wear with underinflation - if the driving is mostly high speed highway driving, the centrifugal force throws the center of the tire outward (remember, not enough pressure to hold a good shape) and wears it more quickly. Conversely, you get the normal heavy shoulder wear with a low-pro tire if the driving is mostly slower or stop-and-go. With low-profile tires though it's often a mix of awful tire wear, rather than one of these extremes. Low profile tires have to have their pressure EXACTLY correct 100% of the time, FYI a very important warning.

And since it comes down to tire pressure, always run pressure in the low-30s, with a few more psi in front than back. Mid 30s for highway driving, closer to 30 for general around town driving.