Question about tire sizes

[Gorcman]

Yea, i read the tutorial on tire sizes posted by prodigy, but im still a little confused.

In terms of clearance issues, is all that matters is the overall diameter of the tire? because when i read the tutorial, it said that some of the many tires that were pretty close the stock tire diameter had clearance issues.

For example, the stock tires (215/55VR17s) have a tire diameter of 668.3 mm.

A tire of:
225/50R18 = 682.2mm
doesnt have clearance issues, according to the tutorial.

but a tire of:
255/40R18 = 661.2mm

has clearance issues, according to the tutorial.

Why is it that the latter one has clearance issues (despite being closer to the stock diameter), but the first one doesnt? Am i missing out on something?

[cklinh]

You're forgetting about the width of the tire.

[theprodigy79]

You're forgetting about the width of the tire.

That's exactly right... in your first example, the tire is 225mm (8.86in) wide... in your second example, the tire is 255mm (10.04in) wide... that's a big difference to have to squeeze between your fender and your strut.