Measuring Tread Depth

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Big Jim

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This is a question for anyone actively involved in the tire industry.

Has there been a change over the last few years on how to measure tread depth?

I ask because a service manager and one of his technicians told me today that the correct way to measure tread depth is from the top of the tread wear indicators, not the deepest part of the channel. This just doesn't sound correct. They claim this is what is being required these days by DOT.

If you have any documented evidence of the correct way to measure tread depth from a tire maker I would really appreciate getting a copy. I found the article on Tire Rack, and it supports my understanding, but I'm not sure if it would satisfy their need for noncontroversial evidence.
 
I was kind of under the impression that the indicators indicated the minimum tread depth... but this will be interesting to see.
 
According to the NHTSA (Natl Highway and Traffic Safety Admin) there has been no change: tires should be replaced when the remaining tread is 1/16" or less - and the TWI's are built in to show that depth. It even mentions that when the Tread Wear Indicators appear "even", it's time to replace the tire. It sounds like your tire tech's have a clever marketing strategy to sell tires. A few tire dealers around here immediately check the age of your tire and tell you to replace them when they are 4-5 years old.

All that being said - your tires lose some of their ability to handle rain/water when the tread is approx half gone. so.... it's not always bad to err on the side of safety.

btw - more info at: http://www.nhtsa.gov/cars/rules/TireSafety/ridesonit/brochure.html
 
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I am not as heavily involved in tire inspection as I was in previous years, but I do have the official CVSA (Commercial Vehicle Safety Alliance) North American Standard Out-of-Service Criteria handbook in hand, which includes info on the Standard Level 1 Passenger Vehicle Inspection Procedure.

The handbook specifically indicates "NOTE: Measurements should not be made on stone ejectors or tread wear indicators." So, nothing has changed and you still measure from the highest to lowest point in the thead groove.
 
Big Jim,

No, there hasn't been a change.

However, there has been a series of bulletins coming out that recommend that tires be replaced at 4/32nds for wet traction purposes - and that's new. There was even a proposal to change the DOT requirement, but that was never acted on.

I'll bet your guy was trying to make sense of this, while trying to fit that into his old way of thinking!
 
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Originally Posted By: CapriRacer
Big Jim,

No, there hasn't been a change.

However, there has been a series of bulletins coming out that recommend that tires be replaced at 4/32nds for wet traction purposes - and that's new. There was even a proposal to change the DOT requirement, but that was never acted on.

I'll bet your guy was trying to make sense of this, while trying to fit that into his old way of thinking!


Do you have any of those bulletin's?
 
Research Results

I was hoping to find a definite answer either from a tire manufacture or from a government source. When could not, I inquired of the California Bureau of Automotive Repair, and obtained a definite answer for California.

It is in the California Vehicle Code. Section 27465. It can be accessed on line. It takes a bit of reading to confirm the thickness to be "two thirty seconds of an inch" after you winnow out the reference to vehicles over 10,000 pounds. It also says not to measure over "humps", which was explained to me to mean tread wear indicators.

If anyone else has any definite information, I would appreciate it if you would post it.
 
Does anyone here feel that 2/32" of tread is adequate? 1/16"? I don't. Nor do I feel that the legal minimum 4/32" (1/8") for snow tires in this state is anywhere close to adequate?

4/32" minimum for summer or all season tires sounds prudent to me.
 
Originally Posted By: Ken2
Does anyone here feel that 2/32" of tread is adequate? 1/16"? I don't. Nor do I feel that the legal minimum 4/32" (1/8") for snow tires in this state is anywhere close to adequate?

4/32" minimum for summer or all season tires sounds prudent to me.

Yes, I do. I had tires went down to less than 1/32" tread depth during summer here without problem. Actually, the car performed better on dry surface with no tread remaining. But it needs 4/32" minimum for wet highway speed.
 
Originally Posted By: Ken2
Does anyone here feel that 2/32" of tread is adequate? 1/16"? I don't. Nor do I feel that the legal minimum 4/32" (1/8") for snow tires in this state is anywhere close to adequate?

4/32" minimum for summer or all season tires sounds prudent to me.


The problem here is that there is no clear break off point. Traction decreases continuously with reduced tread depth. Whatever minimum suggested is arbitrary.
 
My issue is that they were measuring it wrong, whatever the motivation may have been. How to properly measure the remaining tread is a separate issue from when a tire should be replaced.

Maybe you like others making your decisions for you. I don't.
 
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My issue is that they were measuring it wrong, whatever the motivation may have been.

I understand completely, and I can make a guess at their motivation...they were lying to you to make a sale.

HTSS, yes, in Irvine the only time you'll have water on the road in the summer is when somebody is overwatering their lawn. You location is a special case.
 
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