Tread depth

[Chas Henderson #28127]

The tires on my 71 coupe are about eight years old and 32,000 miles on them. I have 6/32 tread left on all the tires as they have worn quit evenly. When new, they had about 11/32. So, that means that I have used up about one half the tread on them. I know that they have wear bars and that they show up with 2/32 tread left, what is a safe depth to drive on? They are Goodyear Eagle ST P225X70R15.

Thanks,
Chasman

[Joe L.]

Re: Tread depth

Chasman----

Usually, tires will be safe down to the legal limit (when the tread bars appear). However, the legal limit applies (at least, in California) when the tread depth is down to the legal limit on 2 adjoining grooves---the bars do not have to appear across the tread, just across 2 grooves and the tire is illegal. It't probably good practice to start making arrangements to replace the tires well before they get down to the legal limit.

By the way, the Eagle ST is no longer manufactured, so you'll have to decide on another tire model when you replace. The Eagle GT or GT II will offer you a comparable tire at a relatively economical price like the ST. These tires are only "S" or "T" rated, but if you don't drive a car "hard-and-wild", they'll do you just fine.

I could never understand the logic of wearing out $1,500 per set, Ultra High Performance rubber on a street car that never needs or uses that sort of capability.

[Duke W.]

Re: Tread depth

If you don't normally drive on wet roads you can wear them down to the cord. That's what I do. I just hate throwing away tires that I haven't "corded". However, as the tread depth decreases, wet traction degrades and can become very poor at 4/32s or less ,especially in heavy rain. It's a matter of being aware of the tires' condition and adjusting your driving accordingly in wet weather.

If the car is garaged and doesn't see a lot of UV radiation from the sun, they are probably not badly aged, but you should inspect the sidewalls regularly for cracking - both sides. Also, be aware that as tires age from time effects the compound hardens and will have less grip than when new. Thin tread from wear and compound hardening from aging can also considerably increase ride harshness, and they will have less puncture resistance when the tread gets thin.

When the time comes you can replace them with the V-rated GY Eagle RS-A, which is one of the three avialable "police pursuit tires". The Firestone Firehawk PV41 and General XP2000V4 are the other two. Check what your local department and state patrol are running on their cruisers! The P225/60R-16s on Crown Vics are also available in size P225/70R-15.

The existing tires are providing good wear, but the hard compound required to achieve this decreases maximum grip. Softer compounds provide greater potential lateral and braking grip.

Duke

[Chas Henderson #28127]

Re: Tread depth

Joe and Duke,
Thanks for the answer. I plan to drive them through this summer and then replace them. I just did not want to drive them down to the wear bars. I would feel safer and I have been caught in the rain at interstate speeds befor. I see that Mid America has a set of tires by Goodyear that are close to what I have now, and these are the tires I drive and get judged on. I rearly go above 80mph anymore so high speed ratings do not concern me.

Thanks,
Chasman

[Duke W.]

Re: Tread depth

As a reminder of what I always advise tire purchasers, regardless of what kind of car they drive or how fast they think they will drive it, "H" or higher speed rated tires have at least one spiral wound nylon cap belt that will prevent catastophic tire disintegation due to a invisible manufacturing defect or overheating due to lack of inflation pressure, which can be caused by a puncture that you are unaware of.

Tires rated below "H", which includes "T" and "S" and non-speed rated tires, do not have these spiral wound nylon cap belts and are much more prone to sudden, catastrophic failure. Several hundred people were killed as a result of the Ford Explorer tire fiasco a few years ago, and though the failures may have been caused by a combination of both manufacturing defects and low recommended inflation tire pressure by Ford, and in some cases exacerbated by driver negligence or lack of emergency recovery skills. the catastophic tire failures that initiated the disasters would not have occured if the tires had nylon cap belts.

High speed rated tires are inherently safer, and you should take this into consideration when selecting tires.

I would never, ever install a tire with less than an "H" speed rating with a nylon cap belt on ANY car I own, and recommend that same to all.

At freeway cruising speeds the only thing that may prevent a disaster from occuring to you are those four nylon cap belts.

Duke

[Chas Henderson #28127]

Re: Tread depth

Duke,

Is the speed rating on the side of the tire? What type of system do they use when an "H" is lower than "S & T but is the better tire - speed wise?

Chasman

[Duke W.]

Re: Tread depth

The speed rating is in the "service description", which follows the size.

For exmaple, the police pursuit tires we are talking about are 225/70R-15 102V.

The service description is "102V". The "102" is the load index, which is a symbol to designate the maximum load. You need a table to convert this to pounds, but the actual load capacity at maximum cold inflation tire is placarded elsewhere on the sidewall. "V" is the speed rating which is 149 MPH.

The current common automotive speed ratings are:

S 112 MPH
T 118 MPH
H 130 MPH
V 149 MPH
W 168 MPH
Y 186 MPH
Z over 149 MPH, (included in tire size may be modified by W or Y in service desctiption)

Don't ask my how the speed ratings are "out of sequence" relative to the alphabet sequence. I don't know!

Z-rated tires are a a little different. The "Z" is in the size, such as 275/40ZR-15, and if there is no service description they have no defined upper speed limit, but most are modified with a W or Y in the service desciption. Visit www.tirerack.com for a complete explanation of speed ratings, service descriptions, and UTQG ratings. Understanding all this data, which is placarded on tire sidewalls and available in on line tire spec sheets will help you make more informed tire purchanse decisions.

You can also look at my tire articles in the Spring and Summer 2003 issues of The Corvette Restorer for a very thorough discussion on Corvette replacement tires and tires in general.

Duke

[Chas Henderson #28127]

Re: Tread depth

Thanks Duke.

Chasman