I just read an interesting article in Rubber & Plastics News (a trade journal for the rubber industry), on tire tread depth.

Basically, it said the old penny in the tread trick was inappropriate and that street tire performance deteriorated significantly when tread depth was less than 4/32" (about 3.2 mm). Keep in mind we are talking street driving and not racing.

Main points:
1. There is no national standard for minimum tread depth for passenger cars (there are laws for OTR trucks).
2. Cars running on tires with 3 mm tread stopped 25% faster from 50 MPH compared to the same cars running on tires with 1.6 mm tread.
3. At 74 MPH, 2/32" vs. 4/32" took an extra 130 feet to stop.

The article said that 2/32" was probably adequate years ago when tires were taller and narrower. Now that most tires are wider and lower profile, the rule needs to be changed.

I've always thought that 2/32" (the approximate depth shown by the penny trick) was inadequate. Now, there is some scientific data to prove it.

The article went on to say that the NHSTA (National Traffic Safety Administration) denied a petition to increase the acceptable minimum tread depth because they thought it was anti-consumer. They said it reduced the value of a tire because it shortened tire life and would generate more scrap tires.

I think I'll be replacing my tires before they get down to 2/32".

I've never been one to run tires that low. In the rain their performance or the lack of it is very eye opening. One other aspect of old tires is the compound changes & gets harder causing the tire to have less stick.

I second what Crashton said, the wet/snow performance of a tire could become terrible even before you get down to 2/32th left. The 175/65/15 stock conti tires I had were hydroplaning at medium rain level at just 60mph, and that was with about 4/32th left, not good at all.

I refer to that kind of snow tire as a hockey puck. We all know what they do on ice.

if low tread is bad, how about racing slicks stopping power?

That was always confusing to me.

That is some amazing info how low tread can give 130 more feet to stop!!

ew

For a racing tire or slick, it's all about the compounding. Some are almost as soft as a pencil eraser.

Your confusing thread with tire stickyness. Compound.

I guess that means those A008s aren't a good deal

i like my tires (on my weekend cars) the best when about 1/2 worn.

Shaved to racing depth.

First set of tires I bought for my race car had 6/32 of tread. The fellow at Blackburn racing said he'd shave them for free down to 2/32. I thought about it & said no thanks. My reasoning being that they'd last longer. Starving racer you know. Well those were the slowest darned tires, darn that tread squirm & they chunked so they lasted not half as long as a set of shavers. Lesson learned, too much tread is slow on the track. More tread is good in the real world.