Tires: tire compatibility, Tire Size Compatibility

Question
Barry,
I recently purchased a 1995 2 wheel drive Chevy Suburban that has Michelin 235/75 R 15 tires on the Rear and Firestone LR 78 15 tires on the front--are these two tire sizes compatible and will this combination work?

Answer
Dave,

The best situation is to have 4 identical tires - identical meaning size, make, model, and state of wear.  This is so the front and rear of the vehicle behave in a coordinated way.  

The combination you have is about as bad as it gets.  And while these 2 tire sizes are pretty close, they are enough different not to recommend continuing to use them.

But there is another consideration.  

Recent bulletins from the tire industry indicate that tires degrade simply due to time.  The age of a tire is important even if the tire is unused.  There some disagreement over how to best express this age limitation, but my take is:

If you live in a hot climate (AZ, CA, NV, TX, and FL) then the limit is six years.  If you live in a cold climate (MN, ND, WI, MT, etc), then the limit is 10 years.  States in between are  ..... ah ........ in between.

Here's how to tell how old the tires are:

First locate the letters "DOT" on the sidewall of the tire.  Nearby will be the DOT code.  DOT codes are 10 to 12 digits long.  BTW the digits can be numbers or letters.

The first 2 digits are a code for the manufacturing plant.  

The next 2 digits are a code for the tire size.  

The next 3 or 4 digits are a code for the type of tire.  

The last 3 or 4 digits are the date code.  The format is week/week/year/year or week/week/year.  These are always numbers.

Starting in the year 2000, the date coding used was 4 digits.  That means the largest number you should see for the year is 10.  Before 1999 the format was 3 digits.  1999 and 2000 are transition years, so you will find both 3 and 4 digits.

The date code only has to be on one side – and it is permissible for there to be a partial DOT code, so long as one side has the complete code.