Tread depth will affect your stopping distance
Any driver can tell you that you shouldn't be driving with bald tires. Many might not know what constitutes a bald tire, though, and why it is dangerous to let your tread depth get too low.
Your driving performance can suffer in several ways if the tread depth of your tires gets too shallow. Worn tires offer less traction in snowy or icy conditions, and are more susceptible to punctures or air pressure loss. Insufficient tread can therefore have a detrimental effect on your vehicle's handling and increase your chances of a flat tire or blowout.
Tires where most of the tread has been lost will also have longer stopping distances in certain conditions. This condition will make it harder to avoid a collision if you need to suddenly brake due to stopped traffic or other obstacles.
The tread of a tire will allow water on the road to pass through the grooves. Grandview Tire & Auto, an automotive shop in Edina, Minnesota, says this design allows the tire to stay in contact with the road and gain traction.
When the tread is worn, it does not provide a sufficient channel to dispel the water. This condition reduces the traction and causes hydroplaning, where the vehicle essentially glides over a layer of water between the tires and road surface. When a vehicle hydroplanes, you won't be able to steer or brake until the tires regain traction.
Hydroplaning is much more likely when the tire's tread is worn. Consumer Reports found that a vehicle tested in wet conditions began to hydroplane at 40 miles per hour, while tires with full tread began to hydroplane at 43 to 44 miles per hour.
Since a tire with low tread has less traction, it also takes more time for it to grip the road sufficiently enough to reduce a vehicle's speed. The Consumer Reports wet weather tests found that vehicles with worn tires needed three to six more feet of roadway to stop compared to vehicles with new tires.
Vehicles traveling at higher speeds required even more distance to slow down when the tires were worn. The company TireRack tested the stopping distances of vehicles with tread depths of 10/32 of an inch, 4/32 of an inch, and 2/32 of an inch on a wet track. These tests were conducted at 70 miles per hour and used both cars and trucks.
These tests found that a BMW sedan with deeper tread depth was able to do a "panic stop" from 70 miles per hour in 3.7 seconds, coming to a halt in 195.2 feet. By contrast, a sedan with a tread depth of 2/32 of an inch was still moving at 55 miles per hour when it reached this distance, even though the brakes had been applied. It took 5.9 seconds and close to twice the deep tread stopping distance—378.8 feet—to come to a stop. The sedan with a tread depth of 4/32 of an inch stopped in 290 feet, after 4.7 seconds.
TireRack had similar results when testing a heavier pickup truck. A Ford F-150 Super Cab 4x2 pickup with deep treads went from 70 miles per hour to a full stop in an average distance of 255.9 feet, taking 4.8 seconds to do so. The truck had only slowed to 58 miles per hour at this distance when braking on the minimal tread, and needed 499.5 feet and 7.5 seconds to stop completely. With the 4/32 tread, it took 377.8 feet and six seconds to stop.
Another test by Tandy Engineering and Associates had similar results. Vehicles with a tread depth of 11/32 of an inch required 9.7 car lengths to do a panic stop from 60 miles per hour. The distance increased to 12.4 car lengths when the tread was at 4/32 of an inch and 15.2 car lengths when it reached 2/32 of an inch. At 1/32 of an inch, which is below the legal minimum, the vehicle only came to a stop after 18.3 car lengths.
Conversely, shallower tread can actually improve vehicle handling in dry conditions. Consumer Reports says the lower tread depth puts more of the tire in contact with the road, improving cornering as well as braking. This quality makes tread-free tires ideal for professional car racing, but not for everyday driving where you're sure to encounter wet or snowy conditions at some point.
Since 1968, the recommended minimum tread depth is 2/32 of an inch. When the wear bars located across the tread are equal to the tread depth, this minimum has been reached and it is time to get a new tire. Drivers can also test for this depth by putting a penny into the tread upside-down; if there is no space between the tread and the top of Abraham Lincoln's head, the tread depth is too shallow.
Drivers should keep an eye on all parts of the tire to make sure the tread depth is sufficient. The retail store Discount Tire says tread is measured in the center as well as the inside and outside of the tire; the lowest measurement is considered to be the tire's tread depth.
Since stopping distance is significantly affected by the time the tread depth reaches 4/32 of an inch, drivers might consider replacing their tires at this point. A quarter test, similar to a penny test, will let you know if the tread is below this point. If you place the coin in the tread upside-down and the top of George Washington's head is visible, the depth is lower than 4/32 of an inch.
If the tread on your tires is nearing either of these points, be prepared for longer stopping distances on the road. Leave at least three seconds of following distance behind the vehicle in front of you, and increase it to at least six seconds during rainy or snowy conditions. If your vehicle starts to hydroplane, take your foot off the gas pedal until you slow to a speed where the tire can grip the road.