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[Matt]

..... stick with a 285/65/18

[XK N00b]

..... stick with a 285/65/18

Could you define what this means? I'm not sure that the 285/65 is. Is it tread width/sidewall height in mms? ?/?/Rim Diameter

[Matt]

285 = width (basically the tire width) in millimeters. 285 millimeters wide.
65 = sidewall height as a percentage of the width. So 65% in this case. 65% of 285mm = 185mm.
18 = wheel diameter.

You can figure out the overall tire diameter by just adding the wheel diameter and 2 times the sidewall (So 18" + 185x2mm = 32.56").

A QUICK way to determine tire diameter is this:
(First Number) x (Second Number) / 1270 + (Third Number)
So: 285 x 65 / 1270 + 18 = 32.58"

To determine tire width, just convert mm to inches by dividing by 25.4:
285/25.4 = 11.22

QED:
285/65/18 = 32.5x11.2x18