We tested two all season tire models comparing their performance with full and half tread depth.
Based on their warranties, the tires should travel roughly 25,000 to 40,000 miles before their tread reaches the halfway point. To simulate wear without the wait, we used a special tire lathe to shave approximately half the tread depth from three sets of tires, one set for each model. Next we logged 1,000 road miles, then compared the half-tread tires with identical sets of new tires in tests through wet, dry, and snowy conditions.
Actual road wear would have also aged the rubber, reducing grip even further. But differences between our half-tread and full-tread tires were striking:
Winter grip slips. Deep grooves and an array of small slits, called "sipes," help most new tires bite into snow. Shallower tread and worn-away sipes reduced snow grip markedly for our half-tread-depth tires. Accelerating to 20 mph on our snow-covered track took an average of 12 feet longer than it did with the full-tread tires, resulting in nearly 15 percent less snow traction overall.
Hydroplaning starts sooner. The faster you drive, the greater the risk of hydroplaning, since higher speeds allow less time for water to escape through the tread grooves. Shallower tread worsens that situation by allowing more water to stay beneath the tire. Our half-tread tires began to skim over the water's surface at as slow as 40 mph in our hydroplane test, about 3 to 4 mph slower than the full-tread tires. As the chart in All-season-tire wear shows, that represents a nearly 8 percent drop in hydroplaning resistance compared with the same model tires when new.