How do you read the numbers. I am resoring a truck and I understand when it says 31x12R15 it's inches, but why is it that when it comes to 16 inch wheels, they give numbers like 275x75R16? what are the units with them?

If the tire manufacturers would get away from the bullshit inch sizing and go to all metric sizing it would simple task to identify what the sizing markings mean.

Here is a great program to help you figure out tire / wheel sizing questions and many other automotive sizing questions.
It will get you in the ballpark anyways.
[url]http://www.csgnetwork.com/automotiveconverters.html[/url]

I just want to know why the tire manufacturers can't make truck tires in car tire sizes? It would be nice to be able to get an on/off road rated tire for my Nissan Frontier without going to some whacky over sized tire.
And it's not just the Frontier, it's all of the compact and mid sized truck that suffer from this bullshit.

USNJoe, what tire size does your Frontier have right now?  there are numerous sizes out there that not everybody carries, but will do the job you want.  You have to remember the proliferation of sizes have made it difficult to carry everything, but normally it can still be obtained. Striker, the formula is right on, and you can also convert it to inch.  Just divide the mm by 25.4 and it will give you the inch equivalent.  To figure out the height of what you have:tire size lt285/75 t16=  Multiply 285 by 75%.  take the answer and multiply by two.  then divide by 25.4 and add wheel diameter(16).  In this case roughly 32in height.  Width just divide 285 by 25.4.  In this case roughly 11in wide.  I say roughly because both measurements will vary depending on wheel width and vehicle loading.  There are light truck tires available that duplicate passenger sizes, just look for "lt" in the size designation instead of "p".  If an exact match is not available, a slightly taller or more narrow tire should be available, that falls under "lt".  Good luck enimga2yOu in your truck restore.

 -