People used to say that if your tire pressure was too high the tread would wear out in the middle and if it was too low the tread would wear out on the edges.
But now that all tires are radial ply, I guess that isn’t applicable any more?
That’s not right. Radial tires will still wear in those patterns. Overinflation still causes radial tires to buldge, leading to accelerated wear on the middle, and underinflation causes tires to collapse, leading to wear on the edges, just like bias-ply.
It still applies to radial tires. Any tire that’s overinflated will still wear out faster at the center of the tread, and any tire that’s underinflated will still wear out on the edges of the tread.
Tester
What they said. The effect is reduced some, but it’s still there.
+1 to the preceding comments.
While radial tires are infinitely superior to the old bias-ply tires, the same principles apply to inflation and tread wear.
Inflating tires (whether bias-ply or radial) to the pressure recommended by the vehicle’s manufacturer is what you should aim for, but you might want to increase the vehicle mfr’s recommended pressure by ~3 lbs, in the interests of better handling and slightly better fuel economy.
However, once you go way over or way under the recommended pressures, your tire wear will suffer and your handling/road holding can be…unpredictable and dangerous.
Al, who told you that overinflation does not wear out the middle of the tread on radial tires?
In any case, they are incorrect
I have seen many radial tires that were extremely worn in the middle, due to overinflation
"But now that all tires are radial ply . . . "
Bias ply tires haven’t been the norm in quite some time
Except for classics and some muscle cars
Just like everyone else said, increasing inflation pressure does cause the center of the tires to wear more - and the effect is small in radial tires.
But in particular, the effect is smaller than the affect other things have on tire wear - like the difference between steer tires wearing out the shoulders and drive tires wearing out the centers. Or the affect camber or toe in have. Or the effect hard cornering has.
Plus the tire manufacturing have been working to dial out pressure sensitivity.
And one last thought: while there isn’t much of an affect, it doesn’t mean you can neglect your pressure. There is a safety issue here. Underinflated tires are much more likely to fail.
So you don’t need to worry too much about getting the pressure EXACTLY right, but you should check it regularly.
Absolutely still applicable.
When I was young we had an additional saying; if your tires aren’t wearing out on the edges first, you aren’t cornering fast enough! 
"When I was young we had an additional saying; if your tires aren’t wearing out on the edges first, you aren’t cornering fast enough! "
Also known as ‘chicken strips’…
Getting back to air pressure. I struggle with that all the time and my snows always seem to wear in the middle faster. There are dramatic temp swings at times from 35 degrees during the day to near zero night to early morning. I make sure it’s 32 psi for the night and morning going to work and during the days and weekends traveling south, it coud be a 40 degree swing in times I put the most miles on the car. But, I have this fetish about erring a little on too much then too little, and I pay for it.
I’d rather err on the high side of pressure rather than low. Low pressures can cause the tire to fail and the handling will be mushy and imprecise. Higher pressures will improve mileage a little, handle better and “crisper”. I’m OK if the center wears out just a little faster. +2 psi over recommended is usually what I run for all those reasons. Radials will be quite happy with that little extra pressure.
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Thank you, I feel better. ;=)