:

According to a recent test completed by online tire retailer The Tire Rack, the famous "penny test" method of checking minimum tire tread depth just doesn't cut it anymore.

Famously, the penny test (pictured) involves inserting a penny into the tread of a tire with President Lincoln's head upside down. If any part of Lincoln's head is covered by the tread, there's at least 2/32 inch of tread depth left -- the minimum legal amount required. The Tire Rack now recommends using a quarter instead. Inserted the same way, if any part of President Washington's head is covered by tread, there's at least 4/32 inch of tread depth left.

To test the effects of tread depth on wet stopping distances, The Tire Rack used a 2006 BMW 325i and a 2006 Ford F-150 Supercab 4x2 for a comparison at a wet-testing facility. Three sets of OEM tires were used for each vehicle: a brand-new set, a set shaved to 4/32 inch, and a set shaved to 2/32 inch. The test area was then dampened to simulate moderate rain conditions. After multiple stops from 70 mph were conducted with both vehicles on all three sets of tires, average stopping distances were calculated and conclusions were drawn.



Source: http://blogs.motortrend.com/6212785/safety/tread-depth-test-out-with-the-penny-in-with-the-quarter/index.html  

Another important consideration a lot of people don't think about is uneven tire wear. I used to be a service advisor and I cannot tell you how many people brought in vehicles with dangerously uneven tires. One way to check is to crank your steering wheel to the right and the left and check for inside sidewall tread separation. Just because the outer tread and sidewall look fine doesn't mean the inside is ok. Plenty of things can contribute to that...alignment, tire pressure, wheel balance. So do this "quarter" test on all over the tread, not just in one place.  

I have uneven tire ware due to modifications to my car too. I go threw tires like crazy. I just cant help it, i love huggin tight corners

Thanks for your insight again !