Expert: Barry Smith - 10/29/2009

Question
Barry,
Is there a chart that gives the circumference of tires. My P>U> has 235-70-16 and I want to no what size 15"tires are the same.

         Thanks Bob

Answer
Bob,

There are any charts like that, but there is a calculator that will help you:

http://www.1010tires.com/TireSizeCalculator.asp

According to this calculator a 235/70R16 has about the same diameter as a 235/75R15.

But that is only half the story.  You also need to consider the load carrying capacity - and you should never go lower in load carrying capacity than the original tires.  The easy way to tell is with Load Index - and that is the numbers in the code known as "Service Description" - the last digit is a letter that indicates the speed rating, and you shouldn't go lower in the alphabet (lower in speed rating).

So a typical 235/70R16 has a Service Description of 104S , but you should check your vehicle tire placard to see what the original tires were.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.  Please check the placard because what I am about to write is dependent on this being correct.

Assuming that I have the correct Service Description of 104S, a 235/75R15 would have to have a minimum of a 104 Load Index and an S speed rating - and such animals exist!