Tires: shaking, minimum tread depth, ford tarus

Question
I got a 1998 ford tarus and my stepdad got tires for it and he "says" that they are new but when I get up to 60 or higher the wheel shakes and sometimes the seat too. And I hear that the treads in the tires should be a surtin depth and the back tires treads are deeper tan the treads in the front. I don't want to get the tires balanced to find out that I need new ones.

Help,
ryan

Answer
Ryan,

New tires will have the same depth if they are all the same size and pattern.  So if you buy a set of new tires they will be identical.  From a vehicle's point of view, that is the best situation.

However, if someone has a set of tires that consist of 2 fairly worn tires and 2 fairly unworn tires, the most worn tires should be on the front.  And if only 2 tires are purchased, the new tires (the deeper tread ones) should be on the rear, to reduce the probability that hydroplaning of the rear tires will cause a vehicle to spin out.  (If the front tires hydroplane, you are less likely to spin out, than if it is the rears).

Having said that, regular tire rotation is really important to prevent irregular wear from appearing, so some compromise in what is on the front is in order.

There is a legal minimum tread depth of 2/32nds of an inch, however there ia a mandatory wear indicator on every tire and when it is exposed anywhere on the tire, the tire has to be removed.  Beware, there are portions of the tread surface where they can't put wear bars, like the shoulders of the tread, and in that case, you'll want to observed if the pattern changes as the tire wears.  When it does, it is probably time to change over to new tires.

BTW, a new tire will show some molding marks on the tread surface, that will be quickly worn away.  Perhaps you should look to satisfy yourself that you are dealing with new tires.  What you should look for are lines or bumps ABOVE the surface of the tread.  That kinds of things that would be worn flat and disappear if additional miles were put on.  If the tires are practically unused, these lines and bumps will be sharp and distinct.

Now to the shake problem.  

Besides balance, there is a tire property called uniformity, which can affect ride.  It is also possible that the wheel is out of round.  Take the vehicle back to where the tires were purchased and make a complaint about the vibration.  Give them the opportunity to fix it.  (BTW, if this vibration was there before the new tires were put on, then the problem is in the vehicle, and different tires may not fix it.)

Hope this helps.