This is probably already known by many but it's still kinda interesting if you dont. So I'll use tire size 295x60R20 for example. If you use a formula (math) like this 295/25.4=11.6 X .6= 6.96 X 2 = 13.9 +20=(33.9") You can also use a slightly different way to do it that's just the way I learned, also many websites have a tool to just plug the numbers in. On a tool I used it comes out 33.9x11.60R20. Now I'll use the Nitto Trail Graps and look at the manufacturer site and they say 34.45x12.13R20= .55" difference from calculation. Anther example is a 35X12.50R20 which comes out to 318/60R20 in a calculation tool which then shows as 35.02. When I looked at the 35X12.50R20 trail grappler on the Nitto site, it shows 35.04X13.39, and for a comparison ill use the Mastercraft courser mxt which shows a diameter of 34.69" and a difference of .33" from manufacturer to calculation. Basically a short version of what I'm saying is, check tire manufacturer also not just the tire calculation, especially if your concerned about fitment. Sorry if you found that very boring, its been a slow day at work.

 

I just go to TireRack and check the specifications for the tire which includes the diameter.

 

For general tech. info they have some good pages, but I'm not really a fan of how the tire search is setup and kinda the whole site in general, and lack of options, at least for the searches I did, specially when it comes to larger then stock sizes.