One of the handiest tools to have is a "Digital Tire Depth Gauge." They can be bought for as little as $25 or as much as $50. I think mine came from toolrage.com buts it's been a while since I bought mine.
On the Prius with Goodyear Integrity tires, I did some measurements of the three primary treads as it came from the dealer who set 34 psi in all the tires. The mileage I checked these at was at 250 miles, so they are relatively new tires.
Front Left: Outer tread depth: 0.273" Middle tread depth: 0.295" Inner tread depth: 0.271"
Front Right: Outer tread depth: 0.274" Middle tread depth: 0.291" Inner tread depth: 0.268"
Rear Left: Outer tread depth: 0.278" Middle tread depth: 0.297" Inner tread depth: 0.268"
Rear Right: Outer tread depth: 0.276" Middle tread depth: 0.294" Inner tread depth: 0.276"
In all instances, the middle tread depth was way too deep meaning the tires are grossly underinflated from what they should be. Some have reported wearing out the sidewalls first which also seems to lead to underinflation. I know some have raised them to as high as 42 psi front and 40 psi back. Seems I need much more air than what the dealer or sticker shows. Downside will be a harsher and noisier ride. :o(
Just a wild test for those interested.......
Carry on.
B~