In wet and wintry weather, all-season tires begin losing traction long before they appear worn out and at a rate that might surprise some drivers, according to Consumer Reports testing. 

Adequate tread depth is essential for tires to grip rain- and snow-covered roads. In CR tire tests, we started to see a decline in the performance of tires with half their tread depth still left.

Snow traction when accelerating fell by 14.5 percent and wet stopping distances increased by about 7 percent, among other findings, when the tires were compared to new versions with full tread depth.

All the tires tested worked well in dry and temperate conditions.

The findings reinforce our recommendation that drivers use winter/snow tires if driving a lot in snow. These are specially formulated and constructed to provide maximum grip in snowy and icy conditions. Drivers should also be aware of their tire tread depth when it's raining.

“Consumers don’t have to replace tires that still have life in them,” said Gene Petersen, who runs the CR tire testing program, “but they should use our test results as a guide to how they should drive in certain winter conditions.”

As part of a broader testing program, Consumer Reports also assesses more than 50 tire models every year, numbering hundreds of individual tires. We put them through more than 14 tests to come up with detailed ratings across many categories. These tests include not only track testing, but also extensive tread wear testing. (See the latest tire ratings.)

How We Tested

To assess the impact of tread depth on winter traction, we compared the performance of three popular all-season tires—one set with full tread and one with only half the tread left.

More on Tires

We shaved tires to approximately half tread depth to simulate a used tire with a lot of life left in it. We tested by accelerating on snow and braking on wet surfaces to assess traction.

We also tracked the speeds at which the tires start to skim or hydroplane on standing water on a road at our Auto Test Center. We also took these tires to a local ice rink to measure stopping distance on ice.

The results show a decline in performance that consumers may experience before the tires are worn-out.

What We Found

For tire geeks, a half-tread depth tire has typically 5/32- to 6/32-inch of tread depth remaining. Tires are worn-out when they reach 2/32-inch, but as the chart below shows, you don’t need to go that low to see a substantial decline in performance on slick (wet, snowy or icy) surfaces.

On average with the half-tread tires, there was:

  • A 14.5 percent decline in snow traction (based on the measured distance that it takes a vehicle to get up to speed)
  • An 8 percent loss in hydroplaning resistance (based on the observed speed at which hydroplaning first occurs driving through standing water at our test track in Connecticut)  
  • A 6.8 percent loss in wet braking (based on the distance to stop from 60 mph on a wet surface, measured at our test track)
  • A small loss in braking ability on ice (based on stopping distance from 10 mph on the surface of a Connecticut ice rink).

When to Replace Tires

To monitor your tire’s tread depth, buy a tire tread depth gauge from an auto parts store, or simply use a quarter to judge whether the tires need replacing. If the top of George Washington’s head is just visible when placed head first in a tread groove, then the tread has about 4/32-inch depth. That’s enough to offer some all-weather grip, but you'll want to start shopping for replacements. Certainly when any one tire groove is at 2/32-inch, it's worn-out and needs immediate replacement. You can check by using a penny—the distance between the top of Lincoln's head is 2/32-inch.

Bottom Line

All-season tires perform well for more modest snow conditions. But as the test results show, if the tires are more than half worn, you may want to wait out the snow storm until the roads have been cleared.

The results also remind that tire performance evolves with use, with dry braking and handling improving slightly over time, but with a greater loss of wet braking, hydroplaning resistance, and snow traction.