If you find aspect ratios confusing, you’re not alone, so let’s simplify it with a diagram. Aspect ratio is nothing more than the relationship between the tire’s sidewall height and the cross section of the tire. The cross section is the width of the tire where it bulges out the most — not the tread width.

As an example, all tires with a 215 size designation have the same section width of 215 mm, or 8.5 inches — but not necessarily the same tread width, sidewall height, or load capacity. So, all 215’s are are not interchangeable. The diagram below shows how four 15-inch, 215-size tires can differ from one another, because of different aspect ratios. Notice how the height of a 215 size tire changes as we go from a 75 series to a 70 series, 65 series, and 60 series tire. Easier to understand?