Every year you take in your car to have a safety inspection performed. But just because your car passes the inspection doesn't necessarily mean your tires are safe to drive on.
By law, a tire isn't illegal to drive on until it reaches 2/32" of tread depth. But unless you live in an area of the world where it never precipitates, you should strongly consider getting new tires far sooner than that.
When your tires were new, they had approximately 10-11/32" of tread depth. The deeper the tread, the more water can channel through the tire without it hydroplaning when wet, and the more surely the tire can dig into a snowy surface for traction. (Taller tread blocks also provide greater isolation from bumps).
In short, tires are worn out well before they appear to be. As with replacing wipers or shock absorbers, the longer you wait, the greater the risk to your safety.
Exhibit A
A popular consumer magazine set up a test to determine how much performance is lost in wet and snowy conditions when using tires worn to 5/32" - 1/2 of the original tread depth. To do so, they tested three all season passenger tires, a category of tire that accounts for over 90% of the replacement tires sold in the U.S. The size tested was 195/70R14, the kind of tire you'd find on your average family car. (The wider your tire, the more of a loss in performance you'll experience as the tread wears).
Testers found
•a 15% reduction in snow traction
•an 8% drop in hydroplaning resistance and
•a 7% increase in braking distance
There were gains in dry weather performance - up to a 4% improvement in braking distance - but the testers concluded: "Better dry grip is not worth the tradeoff you'll make in poor-weather performance, however. The half-tread-depth tires we tested lost far more in wet and snow grip than they gained in dry grip."
Given that a recent NHTSA survey found nearly 1/2 of the 11,500 vehicles checked had at least one tire with 50% tread or less, we suggest you consider checking your tires. There are situations where 7% can mean the difference between life and death.
Exhibit B
Many tires have treads composed of dual layer rubber - the bottom layer is designed to support the top layer, providing stability and improving ride quality. Only the top layer's rubber is designed to contact the road surface. A tire driven on this base layer will have severly compromised traction in all conditions, including dry weather.
Exhibit C
From a 2003 BMW 3series owner's manual:
"Tread depth should not be allowed to go below 4/32in (3mm), even though the legally specified minimum tread depth is 2/32in (1.6mm). Below 4/32in, there is a great risk of hydroplaning, even at relatively moderate speeds and with only small amounts of water on the road."
"Once the tire wears to below 6/32 in (4 mm), winter tires display a perceptible decrease in their ability to cope with winter driving conditions, and should be replaced in the interest of safety."
Exhibit D
From Michelin U.K.'s website: "To guarantee good traction on snow, it is preferable to change your tyres when there is still a depth of 3.5 mm of tread pattern remaining."
Confused by all the fractions? You can measure your tread depth using the change in your ashtray:
•6/32": Stick a penny in the tread - if the Lincoln memorial is fully exposed, you're at 6/32" or less and should replace the tire if you'll be driving in snow.
•4/32": Stick a quarter in the tread - if George Washington's head is exposed, you're down to 4/32" or less and should replace the tire if you'll be driving in rain.
•2/32" Stick a penny in the tread - if Abe Lincoln's head is exposed you're at 2/32" and will need to buy new tires before your next safety inspection.
The real clincher is that tire manufacturers won't honor their treadwear warranties until the tire reaches 2/32" of tread.