The difference between 4/32" and 2/32" of tread depth might not seem like much, but based on research by the Tire Rack, America's largest independent tire tester the difference is significant. For example, a pickup truck traveling at 70 miles per hour that passes the penny test can take up to 499.5 feet to stop on wet pavement . However, the same truck has a stopping distance 122 feet shorter if it passes the quarter test instead. This is a 24 percent difference in stopping distance, and the equivalent of six or more car lengths.
The Tire Rack also reported that tires passing the quarter test exhibited better grip on the road under other driving conditions as well. Given these facts, AAA suggests you put that penny back in your piggy bank and instead use a quarter to check tire wear and determine when it is time for replacement.