A tire size such as 215/75/15 is a metric reading tire size. A tire such as a 31x10.5/15 is a standard reading measuring in inches like it should be(hint hint). In a metric reading, the 215 is the width in mm, the 75 is the profile of the tire(how tall the tire is from the inside diameter to the outside diameter). The 75 means it's 75% of the width of the tire, which is 215mm in this case. All those goofy really thin tires you see on jacked up suv's and sedans and stuff with the big rims are low profile tires. Because they barely have any height from the inside to outside diameter of the tire. Most of them are around 20% or so(of the width of the tire). And 15 is just the rim size. In a standard reading, 31 is the total height of the tire from the ground to the top, 10.5 is the width of the tire, and 15 is the rim size again. I'm pretty sure the biggest a blazer can run without 'rubbing' or mods is a 30" tire, or a 235/75 or so in metric. Hope this clears it up for ya good luck!