I remember when my stepdad showed me how to check tire tread depth with a penny. You stand Lincoln on his head in one of the grooves, and if they’re deep enough to cover the top of his head, you’re all right. If Lincoln’s head has clearance, you need new tires.
That was years ago; I won’t say how many years, but it was way before 2007 when it came out that Washington’s head works better (sorry Abe). According to research reported by Car and Driver, pennies are obsolete. I don’t mean as currency, I mean as tread-checkers.
The quarter test works the same way. It’s just that quarters are bigger, so when you use a quarter you’re measuring 4/32″ deep instead of the penny’s 2/32″. Most manufacturer warranties void beyond 2/32″, and federal regulations require wear indicators at this level. So, you can still drive on tires that pass the penny test. Quarters just give you more wiggle room.
The research stated something of the obvious: tires with healthy treads perform better. The less worn your treads, the safer your ride. When fitted with tires that passed the quarter test, cars traveling at 70mph on a wet road were able to stop 24% shorter than those that passed the less rigorous penny test.
(Granted, it’s generally inadvisable to go 70mph on a wet road, but they did it for science.)
What you should really learn to do (and teach your teen drivers to do) is check the tire wear bars. Part of the tread pattern, these bars go across the voids at a depth of 2/32″. You should replace your tires when the bars become almost level with your tire’s surface. Your mechanic should check these during routine maintenance, but it doesn’t hurt to check them yourself.