Many of us have been doing it for years. When we wonder if our car tires have enough tread to still be considered safe, we whip out a penny, stick it into a tread groove and see if the tread ridge is high enough to touch the top of Lincoln’s head (which you insert pointing down).
In recent years a number of tire experts have begun pushing harder for a new standard for measuring tire tread depth — one that favors a deeper tread than the “penny test” allows. What’s worrisome is that tires with 2/32 of an inch of tread depth, which is very little, still pass the penny test. Some say we should replace tires when the depth is less than 4/32.
This requires changing from a penny to a quarter for the depth test. Tire retailer Tire Rack says people should get used to the quarter test and forget about the penny, mainly because of the way tires perform in wet conditions.
As a tire’s tread wears away, its ability to displace water is reduced. This increases the risk of hydroplaning and a loss of control. Tire Rack’s testers say a too-shallow tread can let you down during the sudden heavy downpours that occur frequently during the summer. Tests have shown that stopping distances on wet roads increase significantly with as tires wear, and the difference between stopping performance of tires with 4/32 and 2/32 of tread depth is eye-opening.
You can see a video about Tire Rack’s tests here.