285/645R18 is between 30-profile and 35-profile.
285/30R18 should give you a tyre diameter of 628.2mm and
285/35R18 should give you a tyre diameter of 656.7mm
so the aspect ratio is somewhere around 33%
My formula for tyre height in mm is:
2*((Section Width * Aspect ratio)/100) + 18*25.4
These would almost be worth buying and having shipped here as the price is actually pretty good, even allowing for freight and import taxes.
RB
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